It isn’t a “proprietary back end” it is what Stallman calls Service as a Software Substitute. (SaaSS) It wouldn’t matter if they claimed it was completely foss. You are still using a foreign service you don’t control.
With a package manager that is sort of unavoidable though. In the case of snaps you could always modify the source to have a different repo. The real reason not to use snaps is all the other issues.
They want you to quit