It's not really just keeping snapshots that is the issue, usually. It's just normal FS operation, meant to prevent data corruption if any of these actions is interrupted, as well as various space-saving measures. Some FSs link files together when saving mass data so that identical blocks between them are only stored once, which means any of those files can only be fully deleted when all of them are. Some FSs log actions onto disk before and after doing them so that they can be restarted if interrupted. Some FSs do genuinely keep files on disk if they're already referenced in a snapshot even if you delete them – this is one instance where a modal about the issue should probably pop up if disk space is low. And some OSes really really really want to move things to .Trash1000 or something else stupid instead of deleting them.
