The biggest problem here
Isn't in allowing people who are allowed to own real guns the opportunity to print out a cheap shitty gun.
The actual problem is in giving people who aren't allowed to own guns (say, ex-cons, children, etc etc) the opportunity to make themselves a shitty gun.
And sure, the gun may be shitty and as likely to injure the person using the gun as much as anything they are shooting at - do we really want to add children injured/killed through misadventure with a shitty printed gun to the number of children injured/killed because of a regular gun being used on them in a school?
Surely the most pro-gun advocate doesn't think children should be allowed to access guns freely and without supervision or training/education - because thats what this will get you.