I don't see anything wrong with this. Charging vs Donations, I don't see what the difference is. It's people paying money for good/services/software. The primary difference is the open ended pricing model.
Google, Apple put up networks and there's value in that network. Whether someone chooses to release their app for free or charge money for it, that app is dictated by the network fees. This network is extremely powerful in terms of its distribution reach. Does it warrant 30%? That's what the market is willing to put up with, but if we're debating 30% - it should be for the whole network not just because someone has an open ended pricing model.
If this app is registered as a non-profit (which it did not look like based on the website?) then I am 100% in the wrong and do believe the app should be able to operate under different pricing schemes.
At a philosophical level - what is the difference between a free-to-use app that has open ended pricing vs a FOSS with donations?
If there is a difference, let's discuss it.