I will be a shooting sports instructor at a local Scout camp this coming summer.
The Rifle Shooting merit badge requirements are strict when it comes to metallic cartridge rifles - they must be .22lr, single shot, or have any kind of ammunition reservoir modified so that only one cartridge can be loaded at a time.
The AR-15 might be suitable for older scouts, but for many boys, whose first experience with firearms is at summer camp, I think these rules are entirely appropriate. Marksmanship is the primary goal - we don't want younger, smaller scouts intimidated by shooting, and we don't want scouts "blasting away" with Rambo-type, action movie delusions in their heads.
Cost, liability, and politics are also issues that have to be dealt with. There are people within the organization who are very concerned with appearing too "para-military" and the "evil black assault rifles" would be a hard sell. Remember that we are often dealing with boys that are children of "soccer moms." There are advantages to introducing these scouts to firearms in a somewhat "politically-neutral" manner.
Certainly, once scouts have shown proficiency with .22 rifles, something like the AR-15 might be an appropriate step if they are interested in furthering their knowledge of the subject. I do not believe there are any specific provisions against the AR-15 or any other type of rifle for troop activities, just not for qualifying for the Rifle Shooting merit badge itself. (If I remember correctly, Philmont offers the opportunity to shoot CMP Garands.)
It sounds like you appreciated the opportunity to learn firearm safety and marksmanship that Scouting gave you. If you think scouts would appreciate the opportunity to try something like the AR-15, you should get in contact with your local affiliates and offer your assistance. [:D]