Doping's one of those classic prohibit / transgress things.
By drawing a more and more specific line, the anti-doping agencies guarantee that:
(a) All competitors know exactly what they should theoretically be doing to maximise their performance within the rules, and they all do it. They game their performance levels in ways that many casual observers will consider to be "against the spirit of fair competition") but because they're within the rules, they can't be sanctioned;
(b) All competitors know their options for breaking the rules and can more easily catalogue ways to work around the "controls" (as you'll hear Tyler Hamilton etc refer to them)
Philosophically, the law is made because it is being broken, and expects to be broken again. The law invites and guarantees transgression - the law defines the dark half of desire and creates a motive where, perhaps, none existed beforehand.
