It's a tradeoff between getting more users and getting more money, which I'm sure he knows. He's chosen to be more towards the "money" end of the scale, which is fair enough.
Stare into the abyss etc.
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.