I don't really see the slippery slope argument. Films are a product that people buy; and the people pledging here are simply buying in advance. People are not being coerced here; they actually want to buy this. I say that as someone with zero interest who will be pledging no money to this effort.
As for why Warner would do this, I think it's because they had little faith in the project bring viable. I imagine that opinion is now changing.