One reason would be the absolute authority given to AA by the courts. Some DUI offenders are sentenced to attend x amount of meetings. That puts AA right up there with prisons and fines in some people's mind.
I see your oint and agree. many in AA resent being used by the courts this way and there are many groups that will NOT sign any court papers. then again I have seen some guys get better because of a court order. It i had to decide I would vote no to court ordered attendance. Asit is though, each group can decide for themselves and so the problem ceases to be a problem for AA as a whole.