Even when my non-res arrives I'll bring buying/bringing 10 rounders to MA if competing. I already have some from the time living there, and in USPSA production ten is the limit anyway, so nothing lost. Yes, buying some limited capacity mags kind of stinks for pistols designed for 15+ but I'd rather spend that $100-$150 or so versus going to court.
I bet a lot of people just figure they'd tell a LEO they were there for a competition and expect to be sent on their way, either out of a belief that it's OK or the presumption that a LEO wouldn't rake them over the coals for something "trivial." And/or that it's not something anyone will notice/care about.
Either that or they think it doesn't apply to them. I once had a MA resident (non-LEO) swear up and down that the 10 round mag capacity law only applied to what dealers could sell in state, and that it was legal and acceptable for him to buy postban high capacity mags in NH or elsewhere and bring them into MA.