I understand the sentiment. Most devs that release Lemmy apps do so as a side projects. They don't make money doing it. In fact, they actually losing money and time to work on their side projects. It's actually amazing how many apps we now have in just a short time.
That being said, Sync developer is one of a few dev who work on a lemmy app full time. This results in higher quality app, even though it's still in beta right now. But since he's literally doing this for a living, he'll need to make some money to continue his work. People was begging for him to make an app for Lemmy, and he actually does it. Whether he'll continue doing this or not will depends on whether he can make a living or not. And since the market for Lemmy apps is so small, there is no other choice but to charge a higher price to make the calculus works. For a lot of people that rooting for him, paying $20 or $17/year is no brainer if it means they get to have their favorite app working on Lemmy.