Going by your point, shouldn't Amazon be blocked since they are collecting the payment info and charging without any interference from Google? How are donation links being considered to be in violation? Just wondering!
The “30% cut” only applies to digital goods like apps, software or music. Physical goods like a book ordered from Amazon (or an Uber ride, for example) are exempt. Amazon-owned properties that sell digital goods are subject to the rule though. That’s for example why you can’t buy Audible audiobooks from within the app.
Both stores require payment through their app stores for digital goods. Neither require it for physical goods (Amazon). The donation is for the software itself and so requires payment via Apple/Google.