Good observations, Orpheum, yet there are plenty of angles to approach this from.
As has been observed here, the MPAA ratings are "guidelines"; opinions of others that you, or your community may or may not agree with. It's easy to pass the buck to them and use their ratings to keep the kids out of R-rated pictures... and I don't necessarily disagree with you on that point. On the other hand, I'd take the other side of the argument that we need to be the community's babysitter. That parents were so unaware of where their juvenile children were that the kids could find their way to an R-rated picture, get caught by the parent and the theatre owner gets a face full of righteous indignance is just too convenient. I know this is political, but I take the side of parental responsibility. I kept track of my girls, sometimes to their consternation. I also felt responsible if I learned that something was going on that I didn't know about, which was thankfully not too often... they've been great kids.
On the other hand, I do agree with the notion that adults shouldn't have to endure juvenile antics at an R picture. In our case, we enforced the ratings more for that reason. R-rated pictures required an attending adult. For other films, we didn't let kids under 12 in without an attending adult. We required the adults to accompany the kids... no going off to watch one show while Johnny and Susie are in another. We got some resistance to that, but we also had very little trouble in the auditoriums. People who tended to drop off their kids learned very quickly that we didn't go there, and they found somewhere else to try it.
The money side? Sure, we lost the income from that part of the community. On the other hand, we were frequently complimented for having the place under control.
I agree... forums like this are great places to compare notes, even if you don't agree with everything you see!