It's tats above the neck and past the wrists - which, though my knowledge is *very* old, used to be illegal to perform in at least several states. (Tattoos on the genitalia are also illegal, but that's not germaine to this particular discussion. :-) )
IMO, and I'm just talking here, I'd guess that mostly it's to disqualify people with gang tattoos. That's just a shot in the dark, but I don't think it's a stretch.
I agree with your assessment.