If I pay nothing, I don't pay.

I don't say "I paid zero." or "I paid nothing" unless someone asks the specific question "How much did you pay for that?"

It's obviously a way to nudge people into paying something.

edit: I made an aside about the use of the word 'donation' by cannabis collectives in the U.S. to denote 'payment' (as a way to operate as non-profits and avoid federal problems) as a reference to similarly weak wording used in a nefarious way, but I thought the relationship to the pattern here was weak, so I removed it.

They do serve a purpose though.