No, he isn't being punished too harshly. There are consequences to pay when one is caught red-handed.
Young people of today believe in moral relativity.....if it feels good, do it. There is no respect for rules or the law these days. There also doesn't seem to be any kind of conscience in wrong doing. Phelps only regrets he was caught, not that he was actually smoking contraband.
I believe smoking marijuana does, in fact, effect one's athletic ability.....not to mention it causes faulty "thinking".