There is no legal requirement to not use the full auto M16 bolt carrier in a semiauto AR-15. The ATF has ruled that doing such does not even begin to make the rifle fully automatic.
Colt (at least in the past) had gone to great lengths to ensure their carriers couldn't be used in a full auto conversion (cutting out the entire bottom of the back of the carrier, unshrouding the firing pin), but that's by no means required.
If a vendor is cutting the bolt carriers so they cannot be used in a FA gun, I'd be buying from somebody else. I wouldn't pass by a good semiauto carrier if I needed one and found one at a good price, but I wouldn't buy one that somebody dinked with because they don't know the actual rules at work.