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.
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.
I'm very curious to try it out, and came to comments in hopes of seeing some comments about what it's like. Instead there's a bunch of comments about the pricing situation. So it's definitely affected my chance of giving it a shot.
I'm using it and it's pretty good. I don't really agree with the scheme of 1. Tricking you into paying something. 2. Feeling guilty for entering $0...but the ui is not bad at all imho.