From your description it sounds more like the fault was with the receptionist who took over the role of deciding who needed the session. I think it's interesting that your therapist would then say it was her fault, but not actually apologize. It feels so removed.

I've noticed this weird dynamic with professionals who have office staff that mess up (or who are rude). They protect these people to the ends of the earth even when they are disruptive to clients. Even my therapist--who does not have staff--says it's a thing that happens where the practitioner is good and the front office people are horrid, but the practitioner won't do anything about it.

Anyway, I don't think you're wrong to be upset about this. It's awful to experience being offered something and then have it taken away because someone else is determined more deserving. It's a hurtful message to send to the client who is passed over.