If a banned user creates a sock and devotes himself to making useful edits, without rekindling the fires of disruption, then how does it benefit the project to tracking him down and kicking him off? That sort of thing merely causes frustration, unpleasantness, and more disruption.
Which is why, as far as I know, it doesn't happen. It's always been the case that a banned user can come back with a new account and as long as no-one can tell they're the same person, there is no problem. The only way someone could tell is by illegal checkusers - do you have evidence of checkuser abuse?