Common sense. These aren't sold without NDAs and license to distribute to third parties. They aren't supported by Intel either. The can be bugs present in these chips which never made the commercially released version.
If buying to run for giggles in a lab, fine. But if for a production machine to do work on. This doesn't make any sense at all.
Agree, I would never use ES versions myself.
But I did not find it common sense that buying ES would be considered a crime. There's a difference between shooting a person and breaching a contract. In the first case, the government takes action against you, in the other a private company takes action against you. So when I doubted the illegality of buying ES samples, it's because I considered illegal to be "stuff that government puts you in prison for".
The case with Gizmodo and iPhone comes to mind. Common sense would say that what Gizmodo did was illegal, but even in such a high-profile case where the buyer obviously knew it to be stolen goods, the public prosecutor gave up pressing criminal charges.