I think that therapists can take care of themselves. If the therapist wanted to bill you for it - he would have done so. I think that if it was a one time accident sort of thing, I would not worry about it. I believe in cancelling if one is not going, but a one time misunderstanding about the time is not, in my opinion, a big deal.
I would very much want to know what the cancellation policy was - I don't like those people being arbitrary at me. But I particularly would not worry about it if the therapist did not give you a policy on it and I would not pay in any case until the time after the policy was stated. I also get to decide if the policy is one I find reasonable. If not, then I would find a new therapist (for example - I would never hire one who thought I was paying them a salary and expected me to pay even with reasonable notice)