When I lived in MA (Brrrrrr), I didn't have a front plate but never got pulled over for it (I did get some blatant DWBs but that's another forum altogether). I asked a police officer once and he said something to the effect of: MA doesn't require cars bought out of state to retrofit front plates, and so they don't ever bother to do anything about it. I suppose if one is pulled over and asked about it, a proper response might be something like "I got this car from another state" and then blink innocently.
YMMV, IANAL, IMHO, LOL.