why would they supply something for use on their network which was capable of doing something they don't like?
Maybe because they can't source any equipment with the granularity of control they would like. Maybe mobile phone technology changes so rapidly its easier to stick it in the AUP.
To be honest, your argument is rather stupid anyway. Its like saying your ISP supplies you with an internet connection that you can access kiddie porn on. Sure they could try to block it.... but then that would put the onus on them to block 100% of it..... much easier for them to state in their AUP that "thou shalt not use our connection to browse illegal content".