Attention Yacht Experts...Your help is needed!

Hi everyone.. I have been reading this forum for a while now and its great. But there is one thing I have been wondering fact it is driving me crazy and I have not found an answer. I see alot of yachts with " Not for sale or charter to US residents while in US waters " ...or some form there of.. and I cannot for the life of me figure out why. I have searched for the answer and have so far come up empty. If anyone knows why some of these yachts are not for sale to people in the US...please tell me! Thanks!

~ C ~