First of all, this excuse implies that since the people in those other sports abuse their horses, it's okay for you to do so as well.
More importantly though, just because there is abuse in every discipline does not mean that they are all created equal. They are different sports! Here is how I decide on my general attitude toward a certain discipline:
1. How common are abusive practices in the sport? Is there one abusive rider/trainer for every 20 good ones or is it the other way around?
2. Have the abusive practices always been part of the sport, or are they recent developments? (Sports are subject to trends like everything else). If they are/were a recent development, how long did it take the sport to recognize that it had a problem? Has any action been taken?
3. How does the governing body of the sport handle outside criticism? Do they ignore it or keep insisting that people just don't understand them? (Or just keep insisting that they "love" their horses?)
4. What kind of rules are in place to protect the welfare of the horses? This includes banning certain types of bits, etc. How effective is the enforcement of those rules?
5. Are there more humane alternatives to the abusive practices in the sport? If yes, why aren't they being used?
6. How open are the participants in the sport to change? How educated are they about equine science?
I'm really not a fan of competitive dressage; I don't think the art of dressage was ever meant to be a competitive sport. However, it DOES pass this test. The majority of dressage riders are not abusive. It may have taken a while, but we did get action taken on rolkur. Totilas' trainer (I forget his name) has been widely criticized for not giving his horse turnout. I don't like Western pleasure either, but from what I've heard attitudes toward peanut rolling have shifted in the sport, so I have to give it some credit as well. My attitudes are based on logic, not whether I personally like the sport or not. (Speaking of which, I also think that one of my favorite sports, show jumping, currently has some major problems).
As for exactly what constitutes abuse, I have only two simple criteria:
1. Does the horse have the opportunity to make whatever is being done to them stop by doing the "correct" behavior? If the answer to this is "no," whatever is being done to the horse is abusive even if it doesn't cause physical damage.
2. Have gentler methods been shown to work to make the horse do whatever it is the person wants them to do?
I think most people would agree with both of those.
Yes there is sometimes controversy in every sport, but there are also reasons why some sports are more controversial than others.