It's a relationship and there are two people. Each person has the right to end the relationship at any time, for any reason.

We speak as though it is all right for a client (due to their issues) to not show up for therapy; complain if they have to pay for sessions for which they did not call and cancel, giving at least 24 hours notice; and leave mid-session or abruptly quit but that it is not all right for the therapist to not see the client anymore and not discuss it/tell the client face-to-face.

It is not mature or "good" social behavior on either parties' part to end the relationship abruptly but I think therapists "get terminated" far more often than clients do.

I think abruptness is not termination, it is quitting or getting fired, depending on which side is ending the relationship. Termination is what happens when the relationship comes to an end by mutual agreement, both parties discuss the relationship and decide to end it, because it is not working for one party or the other. The verb, "terminate" as used in therapy is not meant to be used as a weapon of mass destruction