That's the question (at least for me) here: why are (non-tax-exempt) donations not allowed, beyond the tautological response of "because the rules say so"? I'm not familiar with the regulations around this area, so for all I know there's a great non-Google reason for this and Google just wants to avoid the hassle of conforming to some law. Or maybe donations is a feature they want to eventually support in Google Play and they don't want to have to force a bunch of apps in the future to switch once they add it (which would be a more disappointing reason). Perhaps they're afraid that it would create a loophole where people could get around the 30% cut since psychologically it feels shitty for someone to middle-man a donation, but then if they made donations Google-tax free, everyone would just choose "donation" and they'd have to police this.