Perhaps that's the relevant piece. He would have given it to an existing client who otherwise has a standing appointment but needed a different time this week. I think I could understand that situation because overall, the T would not earn twice from the same time because the standing spot of this other client would then be unused anyway.

You could think about it that way - the other person is paying for their time but they cannot attend their own session time at this week. You are paying for your time but you are cancelling. Thus, the other person comes at your time while no one comes at their time.

I think in those circumstances it would not be unethical. I would consider it unethical if the T would cram a totally new person to the now free session time, effectively earning two times for the same spot.

Although I don't think what your T was thinking and planning was unethical per se, I think he should take into account your preference when making these decisions. I think it is perfectly reasonable to want that if you are paying for your spot that it not be given to someone else, even if you are not attending and even if someone else who cannot come at their regular time would like to reschedule for that spot that week. However, that means that you yourself cannot be expected to be able to reschedule too if for some reason you need it sometimes because then you should expect that other people do not like you to come at their spot too, even if they are not themselves using that time that week.