Actually important NDAs do this - they'll just pay you to come in (or at least not work anywhere else) until the knowledge you had is irrelevant... and if the action that the NDA would prevent is legitimately damaging there do exist laws around corporate espionage that already cover these breeches. If you outright steal legitimately protected information from an employer (a common example is customer lists) and resell it... an NDA isn't needed. NDAs have traditionally just been used as a scare tactic to contain information that isn't legitimately protected... and, of course, to unfairly punish ex-employees for leaving.