One of the problems is that for most live music venues the real profit is to made from bar sales, not ticket sales. Unfortunately that means that there isn't much incentive to host all-ages or alcohol-free shows.
Sticking with the Minor Threat theme: When I was in college i was involved in organising a Fugazi concert. One of the conditions from Fugazi was that there had to be an afternoon alcohol free all-ages show. The venue wasn't interested so the gig didn't happen.
In my (limited) experience it's not unusual for a venue to offer their facilities for no charge in exchange for bar profits. In venues that have no bar, booking can be prohibitive and costs may only be recouped if a bar is put on by the concert organisers - its a bit of a vicious circle really