Racing your horses is not evil.
Letting your horses have a drink, even when hot, is not evil.
It sounds to me like these girls either have bad manners, or don't understand riding ettiquette very well. I don't think it is as horrible and shameful as you are thinking.
I race my horse/gallop flat out on nearly every trail ride. They love it, it's fun, it helps increase their fitness, it helps them blow off steam... All good things in my book. And yes, GASP, i've even galloped in mud. The cow that's making for the gate at a hundred miles an hour doesn't stop to think about how slippery the mud is, you just get in frotn and turn it. Never has my horse slipped on me because of it. It is NOT going to kill me horse. If my horse is thirsty, they get to drink. They don't drink icy water, they don't drink gallons, but if i'm riding all day in hot weather and doing a lot of galloping, its cruel NOT to let them drink.
Now. That being said, it is rude and bad manners to race up behind someone. It is an unspoken rule of the horse world that you treat every unknown rider as a begginer, which means being considerate.
Drinking large amounts of water... Where their horses hot and sweaty? Did they just stop after they let them drink and stand? If the answer to these questions is no, then it really isn't a disaster. I let me horses drink when hot and sweaty, and all I do is make sure to keep them walking for a few minutes afterward, then continue on my ride. Never had a problem.