I believe they're limiting purchases to limit their liability in the event that some nut bar that buys a huge cache from them and then goes and uses it to perpetrate some mass killing.
This way they can say "hey we ONLY sold him 3 boxes, don't sue US!!"
Just thinking litigiously, you know how some Americans can get, all some people want to do is look for someone to sue.
SIG 1911 .45 ACP
SIG P229R E2 SRT 9mm
Beretta BU9 Nano 9mm
Beretta 84FS .380 ACP
PWS (Primary Weapons Systems) MK114 5.56 NATO