I've been seeing this recently on bitcoin job advertisements. For instance: https://bitcoinerjobs.com/job/1288363-mobile-app-developer-synonym

I'm guessing that having US citizens/residents working for a company exposes the company to some kind of regulation risk. But can anyone say specifically what the problem is?

I would have thought they would just want to avoid US customers, not necessarily employees. I know that's the case for a lot of banks outside the US - they won't open bank accounts for Americans, because of the onerous regulations (FATCA compliance).