I have no desire to dive into the larger, deeper discussion here, but in the case of the trainer who has posted in her own defense on this thread, I am honestly trying to look at both sides of the issue - because there are at least two sides and probably more.
On one hand, we have the OP’s account of the incident which doesn’t match the trainer’s - not surprising - especially since one of the two was there, and the other wasn’t. Beyond the incident, we have two different interpretations of what a participant can/can not do on the show grounds after being eliminated. OP’s interpretation of the cited rule is black and white. But EV 138 is specifically about the cross country portion of the event. There may be other rules that address whether the rider can get back on the horse anywhere on the showgrounds after being eliminated, but that wasn’t what was cited.
Having been a keeper of rule books for a much lower level organization, I know it’s very hard to write completely unambiguous rules. Unless it is the same person doing the interpreting/enforcing all the time, there is almost always the potential for different interpretations.