You are right that if a player knows his limits, they would never have a problem with gambling. Excess of almost anything is a problem, be it gambling or alcohol or just anything really.
Anything done that crosses the line always seem to be considered bad behavior that end up harming. Gambling must also have control so that gamblers can prevent many of the problem that exist in gambling, including finance. I have no serious gambling addiction and I have no habit of drinking alcohol or smoking marijuana. This allows me to control the gambling and so far I'm doing well without too many problem.