Is that any different than saying its impossible to compete in track with being dirty? You may not medal if you run a 9.8x time, but you are supposed to be able to compete clean? That would have been a WR not long ago and now it won't get you a medal, but sure, you can compete LIFETIME clean.
Cycling as a sport actually has some testing protocols, which is why you probably find people. The NFL does in-house testing, as does baseball, basketball, etc. If you think they are going to let the star qb/rb test positive for painkillers or some crap like that you're crazy. If you catch a couple, you can make it look like you are actually trying to clean up the sport.