The slab class uses matedata for the 2 states of the block and the 4 different slabs. I believe this basic confuses the code and either you use pickaxes or axes for slabs. I looked at the release 1.3 code to see how they handled this issue and they just made a new block for wooden slabs to be seperate from the stone slabs. I could add this to the mod if people really wanted it, but it would take up two block ID's just to be able an axe on wooden slabs.

public class mod_RespectiveEffectiveness extends BaseModMp {

	public static final int zero = 0;

	public static final int one = 1;

	public static final int two = 2;

	public static final int three = 3;

	public static final String pickaxe = "pickaxe";

	public static final String axe = "axe";

	public static final String shovel = "shovel";


	public mod_RespectiveEffectiveness() {
		MinecraftForge.setBlockHarvestLevel(Block.stone, pickaxe, zero);//1
		MinecraftForge.setBlockHarvestLevel(Block.cobblestone, pickaxe, zero);//4
		MinecraftForge.setBlockHarvestLevel(Block.oreCoal, pickaxe, zero);//16
		MinecraftForge.setBlockHarvestLevel(Block.glass, pickaxe, zero);//20
		MinecraftForge.setBlockHarvestLevel(Block.dispenser, pickaxe, zero);//23
		MinecraftForge.setBlockHarvestLevel(Block.sandStone, pickaxe, zero);//24
		MinecraftForge.setBlockHarvestLevel(Block.rail, pickaxe, zero);//66
		MinecraftForge.setBlockHarvestLevel(Block.railDetector, pickaxe, zero);//27
		MinecraftForge.setBlockHarvestLevel(Block.railPowered, pickaxe, zero);//28
		MinecraftForge.setBlockHarvestLevel(Block.pistonBase, pickaxe, zero);//33
		MinecraftForge.setBlockHarvestLevel(Block.pistonStickyBase, pickaxe, zero);//29
		MinecraftForge.setBlockHarvestLevel(Block.pistonExtension, pickaxe, zero);//34
		MinecraftForge.setBlockHarvestLevel(Block.pistonMoving, pickaxe, zero);//36
		MinecraftForge.setBlockHarvestLevel(Block.stairDouble, 0, pickaxe, zero);//43
		MinecraftForge.setBlockHarvestLevel(Block.stairSingle, 0, pickaxe, zero);//44
		MinecraftForge.setBlockHarvestLevel(Block.stairDouble, 1, pickaxe, zero);//43
		MinecraftForge.setBlockHarvestLevel(Block.stairSingle, 1, pickaxe, zero);//44
		MinecraftForge.setBlockHarvestLevel(Block.stairDouble, 3, pickaxe, zero);//43
		MinecraftForge.setBlockHarvestLevel(Block.stairSingle, 3, pickaxe, zero);//44
		MinecraftForge.setBlockHarvestLevel(Block.brick, pickaxe, zero);//45
		MinecraftForge.setBlockHarvestLevel(Block.cobblestoneMossy, pickaxe, zero);//48
		MinecraftForge.setBlockHarvestLevel(Block.mobSpawner, pickaxe, zero);//52
		MinecraftForge.setBlockHarvestLevel(Block.stoneOvenActive, pickaxe, zero);//61
		MinecraftForge.setBlockHarvestLevel(Block.stoneOvenIdle, pickaxe, zero);//62
		MinecraftForge.setBlockHarvestLevel(Block.stairCompactCobblestone, pickaxe, zero);//67
		MinecraftForge.setBlockHarvestLevel(Block.lever, pickaxe, zero);//69
		MinecraftForge.setBlockHarvestLevel(Block.pressurePlateStone, pickaxe, zero);//70
		MinecraftForge.setBlockHarvestLevel(Block.button, pickaxe, zero);//77
		MinecraftForge.setBlockHarvestLevel(Block.ice, pickaxe, zero);//79
		MinecraftForge.setBlockHarvestLevel(Block.netherrack, pickaxe, zero);//87
		MinecraftForge.setBlockHarvestLevel(Block.glowStone, pickaxe, zero);//89
		MinecraftForge.setBlockHarvestLevel(Block.oreIron, pickaxe, 1);//15
		MinecraftForge.setBlockHarvestLevel(Block.blockSteel, pickaxe, 1);//42
		MinecraftForge.setBlockHarvestLevel(Block.oreLapis, pickaxe, 1);//21
		MinecraftForge.setBlockHarvestLevel(Block.blockLapis, pickaxe, 1);//22
		MinecraftForge.setBlockHarvestLevel(Block.doorSteel, pickaxe, 2);//71
		MinecraftForge.setBlockHarvestLevel(Block.oreDiamond, pickaxe, 2);//56
		MinecraftForge.setBlockHarvestLevel(Block.blockDiamond, pickaxe, 2);//57
		MinecraftForge.setBlockHarvestLevel(Block.oreGold, pickaxe, 2);//14
		MinecraftForge.setBlockHarvestLevel(Block.blockGold, pickaxe, 2);//41
		MinecraftForge.setBlockHarvestLevel(Block.oreRedstone, pickaxe, 2);//73
		MinecraftForge.setBlockHarvestLevel(Block.oreRedstoneGlowing, pickaxe, 2);//74
		MinecraftForge.setBlockHarvestLevel(Block.obsidian, pickaxe, 3);//49
		
		MinecraftForge.setBlockHarvestLevel(Block.wood, axe, zero);//17
		MinecraftForge.setBlockHarvestLevel(Block.sponge, axe, zero);//19
		MinecraftForge.setBlockHarvestLevel(Block.glass, axe, zero);//20
		MinecraftForge.setBlockHarvestLevel(Block.musicBlock, axe, zero);//25
		MinecraftForge.setBlockHarvestLevel(Block.blockBed, axe, zero);//26
		MinecraftForge.setBlockHarvestLevel(Block.web, axe, zero);//30
		MinecraftForge.setBlockHarvestLevel(Block.stairDouble, 2, axe, zero);//43
		MinecraftForge.setBlockHarvestLevel(Block.stairSingle, 2, axe, zero);//44
		MinecraftForge.setBlockHarvestLevel(Block.workbench, axe, zero);//58
		MinecraftForge.setBlockHarvestLevel(Block.bookShelf, axe, zero);//47
		MinecraftForge.setBlockHarvestLevel(Block.pressurePlatePlanks, axe, zero);//72
		MinecraftForge.setBlockHarvestLevel(Block.stairCompactPlanks, axe, zero);//53
		MinecraftForge.setBlockHarvestLevel(Block.chest, axe, zero);//54
		MinecraftForge.setBlockHarvestLevel(Block.lockedChest, axe, zero);
		MinecraftForge.setBlockHarvestLevel(Block.doorWood, axe, zero);//64
		MinecraftForge.setBlockHarvestLevel(Block.trapdoor, axe, zero);
		MinecraftForge.setBlockHarvestLevel(Block.ladder, axe, zero);//65
		MinecraftForge.setBlockHarvestLevel(Block.signPost, axe, zero);//63
		MinecraftForge.setBlockHarvestLevel(Block.signWall, axe, zero);//68
		MinecraftForge.setBlockHarvestLevel(Block.cactus, axe, zero);//81
		MinecraftForge.setBlockHarvestLevel(Block.cake, axe, zero);//92
		MinecraftForge.setBlockHarvestLevel(Block.jukebox, axe, zero);//84
		MinecraftForge.setBlockHarvestLevel(Block.fence, axe, zero);//85
		MinecraftForge.setBlockHarvestLevel(Block.pumpkin, axe, zero);//86
		MinecraftForge.setBlockHarvestLevel(Block.pumpkinLantern, axe, zero);//91
		MinecraftForge.setBlockHarvestLevel(Block.planks, axe, zero);//5
		MinecraftForge.setBlockHarvestLevel(Block.blockSnow, axe, zero);//80
		MinecraftForge.setBlockHarvestLevel(Block.snow, axe, zero);//78
		
		MinecraftForge.setBlockHarvestLevel(Block.grass, shovel, zero);//2
		MinecraftForge.setBlockHarvestLevel(Block.dirt, shovel, zero);//3
		MinecraftForge.setBlockHarvestLevel(Block.sand, shovel, zero);//12
		MinecraftForge.setBlockHarvestLevel(Block.gravel, shovel, zero);//13
		MinecraftForge.setBlockHarvestLevel(Block.tilledField, shovel, zero);//60
		MinecraftForge.setBlockHarvestLevel(Block.slowSand, shovel, zero);//88
		MinecraftForge.setBlockHarvestLevel(Block.blockClay, shovel, zero);//82
		
		//unneeded blocks
		//Sapling //6
		//Bedrock //7
		//Water Moving //8
		//Water Still //9
		//Lava Moving //10
		//Lava Still //11
		//leaves //18
		//Tall Grass //31
		//Dead Bush //32
		//Wool //35
		//Yellow Flower //37
		//Red Flower //38
		//Brown Mushroom //39
		//Red Mushroom //40
		//Torch //50
		//Fire //51
		//Redstone Wire //55
		//crops //59
		//Redstone Torch //75
		//Redstone Torch //76
		//Reed //83
		//Repeater //93
		//Repeater //94
	}
 
	public String Version() {
		return "v1.0";
	}
	
	public String Name() {
		return "Respective Effectiveness";
	}

}