It's fine to prefer people give away things, especially in OSS communities - I'd prefer free stuff too, but I don't feel like it's unreasonable to try to charge

It's fine to prefer people give away things, especially in OSS communities - I'd prefer free stuff too, but I don't feel like it's unreasonable to try to charge for it any more than it's unreasonable to ask for donation or have a premium support package for OSS projects. Mastodon itself isn't offering a paid option, this is a third party who's promising a more well-featured and supported application than any of the current free/OSS ones. If someone wants to copy those features into an OSS client, they're welcome to, but they haven't yet.