Look down on the behavior, not the equipment. If they want another hand position, great. If they want to practice safely using aero bars at a low speed on empty stretches of trail, great.

If they're gonna go 25mph on crowded trails on aero bars, well, that's a terrible idea. But it's also a terrible idea to do it on any kind of handlebar.

Don't give the guy in a Corvette a ticket just for having a sports car, give him a ticket if he does something illegal or dangerous. Same goes for bike equipment. If someone's riding in a prudent and safe manner, don't hassle them.