My T doesn't have a blanket policy of not treating offenders. He's been a T working with SA survivors for about 30 years, and during that time he's treated several survivors who eventually trusted him enough to reveal that they had also SA'd others. By then he had already established a good working relationship with them, and he believed that it took courage to reveal that to him and they needed treatment as much as any other survivor, so his policy is to always be aware of the possibility that a survivor may also be an active or past offender. He will not abandon a patient who came to therapy as a survivor but then admits to being an offender, so he works with them from both angles, to help them understand that they were not responsible for what happened to them as children but they certainly are responsible for their behavior as adults.
I like his policy. I think a blanket "no offender" policy can be counterproductive because it clearly gives all patients the message that they cannot be honest about their own behavior if it involves abusing others. To me, that seems to defeat the purpose of good therapy.