You might not care what you signed but a signed contract is binding, that is the agreement you chose to abide by. Also, it is the treatment provider who sets the conditions for how they work, not the client dictating their conditions to the therapist.

Being angry won't change the way your therapist chooses to work. This is where free will comes into play - if you are not happy with your T's conditions, you are free to go to another therapist.

And it is not a T's responsibility to check whether clients can come. This is about personal agency. The client can reach out to confirm but a it is not a T's job to check whether a client can come to an appointment you both scheduled.