Basically you're OK as long as you don't get paid for your work there. My friend works in the oil/gas industry and he keeps flying to different countries frequently and works there for almost 3-4 weeks. But he doesn't get paid in that country (he gets some per-diem allowance but that's all).
Of course, it's also extremely unlikely that they'll get caught (except, perhaps, as a result of bragging about it on the Internet...). A guy, or bunch of guys, sitting on the beach with laptops isn't exactly an atypical sight.
Still, they might want to pick their destinations based less on the quality of the beaches and nightlife, and with more consideration towards those which aren't going to try to skewer them for taxes or visa fees if their scheme gets found out.