Why is this information not required when we're paying for a "pay what you want" game, but IS required if the game has a minimum cost? Is it because it's considered a donation and somehow the laws are different?
Yes.
It's also what other stores like GOG and Steam need, and I don't think they're "breaking the law" by not requiring addresses.
GOG is based in Europe. I don't know what Steam does to get around this requirement, if it's true.