The Cascade Bicycle Club has the following rules for STP:
Single File is Safer and specifies a $25 fine for the following, tickets issued by the Goldwings ride marshals:
Using voice and hand signals/calling out when passing
Move off the road when stopping
Ride right/Pass left
Do not pass too close/ride too close to another rider
Do not hold up more than 5 vehicles
Running through a stop sign or red light without stopping (riders do not necessarily have to unclip, but they must come to an obvious momentary stop) Aero bars are not mentioned. Nor have I ever been on a ride where they were banned, having never ridden in France. Though there may be US rides where they are prohibited.
Passing other bicyclists on the right in a dangerous manner
Riding more than two abreast
Crossing a solid centerline
Public urination or littering
Not wearing a helmet
I've been riding with aero bars (clip ons) for over a decade. They are maligned by those who are not capable of riding them. That being said, they are never to be used except with a clear road ahead, so either solo or pulling.
It's not tacky, and aero bars do not make a bike squirrely. Poor riders make a bike squirrely. I have found that in-ride policing by other riders fixes any problems, just like it does problems with the many other riding offenses by inexperienced or egotistical cyclists. They sure fix the problem of riders making unexpected braking decisions, which is a very common cause of accidents.
Misuse of aero bars is not a special case of poor riding behavior, just another case of same. Hands on bar tops in a paceline is about as bad.
I would call posters attention to the rule, "No headphones/earphones." They are illegal in my state and are much defended by posters on these forums, although they are much more dangerous than aero bars. Your eyes and ears are your best defenses.