I also don't think you're overreacting. I know my ex-T and current marriage counselor technically have cancellation policies where they can charge for last-minute cancellations, but for the few times I/we've had to cancel, for our own or our daughter's illness, they haven't charged. And once, I was in the waiting room for ex-T, and one of MC's clients showed up--apparently half of a marriage counseling pair. She said her H wasn't coming because he was in the ER with bad back pain. MC was like, "Get out of here, go be with him." And she said she felt bad because they'd missed another appointment and thought he might charge them, and MC said, "I wouldn't charge you for this!"

I would definitely bring it up though. My current T also has a policy for charging for e-mails/texts/calls that take over a certain amount of time. I had thought he'd said occasional longer e-mails are free/that he'd warn me before charging. But then he charged me for one. I told him I was upset and about how I'd understood what he'd said. He chalked it up to a misunderstanding and removed the charge from my account. Had I not brought it up to him, I probably would have stayed annoyed for a while, which would have negatively affected the therapy. It was a good way to find out, early on, how he handles disagreements with clients.

Out of curiosity, is your T a part of a practice, where you're billed by an administrator/receptionist? If so, it's also possible that it's automatic to bill clients for late cancellations, and she may not have even been aware they were charging you. Either way, definitely talk about it.