Yep, illegal to utilize any drug to alter the behavior or attitude of a horse. From the way I read it, it should also be illegal as far as HYPP goes but they probably made an exception to that one. They shouldn't have, however. That would keep people from showing HYPP+ horses and thus help to wipe out the disease.
Unfortunately, not everyone shares your opinion on cheating. Too many people are in it only for the win and will do whatever necessary.
And as far as it being easy, it says that horses are subject to testing, not that they will be tested. I understand that to mean that if there is a complaint, they will test a horse or if they are suspicious they will test. Just on a regular day with no suspicions or complaints, they probably only test 1 horse out of every 100. Inject the horse shortly before the show and be descreet about it, inject enough to subdue him but not enough to make him look lethargic and no one would ever suspect. How would they tell him from all the other horses plodding around hanging their heads?