USADA doesn't mean a completely clean sport, just like physicians don't erase diseases. USADA is there to limit doping and limited doping is far superior to no drug testing at any possible level.
That's really not hard to understand, so why do people keep mentioning such lame and useless arguments ? So physicians are useless since they don't erase diseases, right ?
Pride FC was a complete circus while UFC under USADA was a legit and professional organization.
As for Fedor in the USA, he had to face some drug testing which is still better than nothing, even if far less inferior than USADA.
But that's not the main point anyway : when he fought in the USA, MMA was already a bit more developed, so he got quickly exposed. He simply faced better competition than in his japanese circus show.