Hm, my T prefers clients not to be under influence too but I can't imagine him kicking me out of session if I was (unless I was being aggressive). That's just unhelpful.
Did she tell you this was her "rule" for therapy. My T stated in the beginning that if clients are heavily medicated (legally or illegally), talk therapy is usually pointless. So this eventually could be a reason for terminating or pausing of therapy but only if the T feels he can't help the client and only after a long discussion with the client over this (cause the client could still find it helpful even if the T can't see it and then it could be beneficial to continue). Terminating abruptly (even after only 3 session) is unprofessional.