It’s not about the price (which is 4 beers in my country by the way), it’s about the fact of payment.
What if someone has no payment method to add to Google Play (wrong card type for example)? By the way, all open source and privacy fans are so eager to give credit card info to Google?
What if someone has already donated $5 and doesn’t want to make another donation? Because this feels like forcing a donation… And what if someone wants to donate $10? Two payments would be required.
Last but not least, if you’d rely on the beta app, you could now stand in front of locked door, and they promised it would never happen. You probably shouldn’t rely on beta app (what if you don’t realize), and you probably have access to non-Android device (what if you don’t), but just the fact that they made such step makes you think if they won’t pull something like that in the future - that’s what I mean with ‘trustworthy’.