>What they should not be able to do is poach specific clients. //
I find your indignation interesting as this is exactly what the non-compete would be preventing - the subcontractor from working directly for the client that you (the main contractor) introduced them to.
I was thinking more about the main thrust of non-competes I've come across, which is that you're not allowed to compete in the same industry. I read an article a couple of years ago about hairstylists and other service industry workers being out of a job for 12 months because they'd been forced to sign non-competes.
Not poaching clients is a common clause though, apparently. I haven't really run across that clause because I don't have to power to solicit clients for my employers anyway.
I find your indignation interesting as this is exactly what the non-compete would be preventing - the subcontractor from working directly for the client that you (the main contractor) introduced them to.