Except ... they will put almost everyone on the list (remember Filegate & the IRS scandal). Had they refused to sell him a gun, he'd have sued saying "They refused to sell me a gun because I'm Muslim."
No criminal will obey that law and since gun ownership is a right guaranteed by the Constitution, it doesn't fall under the "privilege" definition most lefties bring up. The fact is, "how you think" isn't a valid reason to deny anyone one of their Constitutional rights.
Besides, do you think the only reason he carried this out was access to guns? That's where the libs just don't get it. He would've created a pipe bomb, a propane bomb, or some other way that could've killed many, many more.