They are afraid of getting sued.

I hate to admit it, but they are doing the right thing. There are many parents out there that'd live to exploit a good-hearted driver for some money.
Still, maybe the school should think about having security guards on the bus.

I would freak ifn it happend to one of my kids.. right now my kids are dealing with some older kids at a bus stop they have to pass on their way to school, so I called the school (both my kids school and the school where the kids at the bus stop attend), and apparenlty its not their problem, both schools told me I have to call the police.. I always thought if it was happening at a bus stop it was the schools obligation to deal with it, or at least make an effort to, granted its not physical abuse or anything as serious as that, at this point its the older kids threating to take their bikes and things of that nature , but still .. as a parent I am not by any means comfortable knowing that my kids have to deal with that every morning.