Just my logic, for what is worthed@

1) some want to cheat
2) because some want to cheat, the body must have tests
3) If a guy fails a test, action must be taken.
4) If he guy does NOT fail a test, we must believe he's clean.

Funny thing is:
some NEVER fail a test, but the public still point them as guilty (Armstrong)
some DO fail a test (or "forget" it: Ohrugu, Ferdinand), but the public still view them as clean.

Right or wrong I trust the tests: fail (or "forget") is guilty, pass is clean.
I know it's not perfect but is the best we can do.