Legit (the Special Forces Association does receive compensation for it's endorsement ), but... IMO they use questionable advertising. AFAIK there is no "US Special Forces Foundation" which they claim ...
"Every USM4 Special Forces lower receiver is 100% made in the USA, completely Mil-Spec, and
fully endorsed by US Special Forces Foundation."
http://www.mpguns.com/usm4/about.php