I am certainly no expert, but I thought that under Federal law it (was) possible to purchase a US-built fully-automatic weapon with the payment a $500 tax (obviously more significant in the 1930s) and the recommendation of one's local chief of police. Many STATE laws additionally prohibit the ownership of fully-automatic weapons. The AK47 is full-fire only, and is NOT US-built, so an AK47 could and can NOT be owned legally anywhere in the US. If Mr. Kerry owns such a weapon he is likely in violation of a variety of state and Federal laws.
OK, controlled, not banned outright. That makes sense. Pretty big camel came in under that nose, didn't it?