TL

“In general it can be said that in the passive voice the subject is acted upon or receives the action expressed by the verb.”

I agree with Mounce. But, you are forgetting something. In the case at hand, the subject is BOTH the actor and the one acted upon. This makes the difference. We are to subject self. We are not subjected by or acted upon by another, which is what Mounce is explaining. Thus, we initiate the subjection and we also receive the action passively. If you do some research I think you will see what I’m saying.