Because it wouldn't be perceived that they were gaining an unfair advantage over their competitors, also music is not a competition.
Whether you like it or not, think its right or wrong, the fact is drugs and music go together. I'm not talking Heroin or Cocaine but Cannabis, speed, Ecstasy etc enhances the experience of playing, listening and writing music and without them most of the music we ALL love to listen to from the past and present probably wouldn't exist.
I'm not saying drugs are good and everyone should take them (far from it), but people that do, do it for a reason. Those that do it to the point of self destruction would probably find another way if drugs weren't available, like Alcohol, its legal but is that really any different/better?
Just for information, I don't take drugs any more because drug testing is regular in the work I've done for the last 13 - 14 years so I chose to sacrifice that for the sake of having a good job and earning good money, so I drink alcohol instead (not at work ) and I enjoy it But if both were legal and I had to choose between drink or drugs, I would choose drugs. In the 8 years I was a regular Pot smoker with weekends spent on speed or the occasional 'e' I NEVER had a bad experience, felt ill, had a hangover, got in a fight, or trouble with the law. I found it enhanced the pleasure in everything I enjoyed, music, films, sex and even the most simple things like laying on the sea-wall and staring at the stars.
I suppose my point is - its not the drugs that is the problem its the people that take them and their attitude to life.
Said like the late great Bill Hicks
The hilarious thing is that if the government could work out how to tax them, drug would be legal!