Apparently the horses are unable to form a union to protect their rights:
For the first time, all 20 horses in the Kentucky Derby were administered surprise tests for performance-enhancing drugs. Waterman said testing has been difficult, in part because the drugs can be detected in a horse's system only three to five days after being ingested, but the effects last longer.
He said there was uncertainty whether three days before the race is the right time to test a horse. But, added Waterman, "It's better than post-race, I'll tell you that much."
So the NFL and MLB need prior permission to test their athletes and aren't forthcoming with the results but Derby officials can (not-so-randomly) test their athletes. In both cases, the health and safety, as well as the image of the sports, are the main reason for the testing. I believe that especially MLB should follow the lead of the horse industry.
Also this weekend, The Fight that "will save boxing" takes place in Vegas at the MGM Grand which has become the Mecca for great fights. I think that Saturday could go down as one of the greatest days EVER!!! Derby, potential for a great fight AND Cinco de Mayo?!?!?! The stars have aligned! (Hopefully the horses will not 'roid rage and attack each other!)