Why are horses disqualified in horse racing?

A horse can be disqualified for many reasons, but by far the most common reason is interference with a competitor.
In the U.S., the basic rule of thumb is that a horse that impedes another runner and costs that horse a better placing can be disqualified. If that happens, the horse that caused the interference is placed behind the horse that it impeded.
The problem is that stewards, the all-powerful authorities who are charged with enforcing the rules of racing at different racetracks, don’t always apply the rule the same way, which inevitably leads to controversy.
Other reasons a horse can be disqualified include failure to “weigh in” or “weigh out” correctly; failure to carry the assigned weight; or being ineligible for the conditions of a race (racing officials usually realize this before the running of a race, but not always). Horses also can be disqualified long after a race is run if lab testing shows it was running with a prohibited substance in its system.