I don't really get how everyone focuses on the ad-free feature as if it's the only thing that Sync Ultra provides. It also provides text recognition in images, translation in-app, (both requiring constant server work) and will eventually support push notifications (again requiring server work). On top of that, LJ has stated he wants to work on this app full-time, which is only possible if he earns a living from it.

If those features aren't interesting to a user, there's always the one-time ad removal option (I'll admit which is a bit pricey but per OP's post, is a one time fee and not a subscription).