I don't think it's an offence as in it's not illegal for a US citizen to visit Cuba, but the minute you spend money there you break treasury department laws under the embargo (there's about 5 or 6 laws applying to Cuba including the 1917 "trading with the enemy" law)!
So a stamp in your US passport (or any passport where the person lives in the US) would make you a prime target for customs and if they could find proof that you bought anything in Cuba then it would be confiscated and in theory as a US citizen you could be prosecuted for breaking OFAC rules, but more than likely you would be fined.
A friend travelled there under a US licensed travel scheme (designed as a "humanitarian" tour) 2 years ago and he politely the perplexed Cuban immigration official to stamp his US passport as he was there legally, and as being there legally they were allowed being back cultural nick nacks (cigars weren't counted as that unfortunately for him)!