I'm not talking about an employee selling them, of even someone dumpster diving (which isn't illegal everywhere). I'm talking about large companies disposing of their old ES chips, and then some of them ending up in other people's hands (legally).
For example, if Dell needs to dispose of some old ES chips, they are unlikely to personally destroy each individual chip. They'll more than likely just toss them in with the rest of their e-waste since they don't really care what happens to them. Dell's scrap will get picked up by a recycling company, and at that point, they belong to the recycling company who bought the load of scrap (or was paid to haul it away). By the time the chips meet their final fate, they will have passed through many hands (non except Dell having any kind of agreement with Intel/AMD). At any point in the chain some of the chips may be sold to people who might decide they're worth more as CPUs rather than melting down.
At no point was anything stolen. The worst crime committed might be breech of contract when Dell failed to personally destroy each and every chip. If Intel/AMD cared so much, they would require that every single chip be returned to them (accounted for by serial number). They would also keep an eye on places like eBay, and have all questionable listings removed.
Having said that, I wouldn't recommend buying an ES CPU since you never really know what it is, or how it may have been abused.