David,

Not an NRA member, but having spent Friday talking to a gun nut friend of
mine I discovered a particular reason that the AR-15 lower receiver might be
on thingiverse.

Apparently different states have different laws regarding automatic weapons,
and lower receivers are heavily customized to meet individual state
requirements.  California for instance has requirements on how magazines are
ejected from rifles that directly impact the design of the lower receiver.
For folks who are gun smiths having CAD designs available on thingiverse is
relevant to their interests.

There are also sort of hacker style armorers out there too.  Such as
Valkyrie firearms in the bay area.

Whether you are into marksmanship or not, some people definitely are, and
thingiverse does pose a potential benefit to their hobby.

And I hope that is clear to you regardless of what you end up choosing to
believe is the right solution here.

Regards,
   Matt