I've paid entries for another amateur to ride my horse, and had my entries paid to ride in an amateur class. Once was because I wanted the horse to attend a specific show that I couldn't get to and the horse had points, another time someone paid my fees to fill a class or any number of other reasons. I've been asked to take one in so that someone else can see it go with an ammy at that height.
I get the idea, but in practice, I'm not sure it makes sense.
Also, I'm completely unsurprised that the USEF can't even interpret their own rules about this.