Collusion between employers to ensure that non-competes are enforced sounds very plausible, given that it is legal.
I think that the existence of a non-compete may also be a liability for the new employer, and it's not solely a practice meant to remind labor of what their place is.
A lot of sub-industries are also "where everyone knows your name" places. I used to be an IT industry analyst (a couple of the large firms in which were known--at least at the time--for being pretty hard-core on non-competes) and pretty much everyone knew what firms other people in their space worked for. You could never have gone to a different firm and had your prior employer not know.