I glanced at the code a bit and I assume that the recipes are intended to accept them. Tried to find why the don't, but didn't find the end of that rabbithole.
Best guess: Bucket gets added only after the recipes are set up.

The issue is with fluid containers not recognized in crafting recipe.

There are some plans to "fix" this with our own implementation of the prior pull request. I'm sure CovertJaguar can explain more though.

MinecraftForge/MinecraftForge#3332

That is intentional. CJ opened a pull request to forge but was not accepted because he failed to maintain it.

oh. strange. okay then :) i mean i tried this in a vanilla + railcraft only setting with the universal creosote bucket that was added by rc so i'm genuinely curious as to what the issue is.

Affects Railcraft recipes as well. This is a bit more important than I thought.

We can create a custom recipes class for this for now.

Fixed in that commit above.