That's my understanding too. I know it's "just customer support" at benelliusa, but I'm sure they have had to answer this question several times.
There is a lot of confusion out there about the M4 and increasing magazine capacity. I like many of you have read countless threads on several forums that have discussed this issue to death. You take all those discussions to a 3-gun match and you'll see that everyone has their own opinion on the matter.
I overheard a fellow shooter at a multi-gun match last month announce to everyone on the stage that, "I guess it's time to take my illegal M4 home because it has a pistol grip and a +2 ext. on it!" The match director and a couple of other mis-informed shooters were the ones that had told him of the problem. The guy went on to say, "I'd like to see that case go to court, I'm sure that I could beat that BS anyday of the week!"
Personaly, I would like to see Benelli step up and issue a letter on their web site that informs any M4 owner or future M4 owner that is in fact alright to add a USA made extension just not an imported one. Would that be asking too much? If their customer support is telling everyone that, then why not issue a formal letter so that this confusion could be cleared up.