USADA has to act within its remit and can be challenged legally.
Armstrong challenged the arbitration from taking place and lost.
He could have defended himself and challenged any aspect of it that he thought was outside its powers in court.
He could even try to challenge the judgement in a court and maybe the UCI will.
For better or worse, there are harsh doping controls for all competitors so its the price you now pay to play sport.
Nadal is to tennis what 4'33" is to music.