There's probably legal concerns on their part as well: gun manufacturers get sued. A lot. By government departments, by victims of gun crime, lawsuits both genuine and spurious. If you're a manufacturer of 3D printers you can probably think of much, much better ways to spend your money than defending or settling a lawsuit against you.

If this guy wants to actually go ahead with his project he's probably going to have to accept he'll need to buy the printer rather than lease it.