How much do you want to bet that shop has a service for assembling your lower by an "ATF approved" store employee?
That would be my guess...
Three word answer as to those signs:
Cite the law.
If they can't do that, I'd cheerfully tell them what to do with a load of sand.
Sadly, there's a local version of "Don's Guns" just about everywhere.
Blessings,
Bill