Part of the purpose of moving from Reddit to Lemmy is that a for profit corporation making money off of the free content of users was considered bad. I might even say this was the main point of contention and thought that spawned the creation of Lemmy. A free open platform where the users are more in control of their own content. Adding ads back into the mix means that somebody is profiting again off of the free content of the users. On principal this goes against the purpose of Lemmy. You may as well just use Reddit as the end goal of monetizing the fediverse is basically going to end up the same.

So for Sync to not be against the spirit of Lemmy it would need to remove its free version with ads so that the only thing you are paying for is the app itself and not for the content without ads.