While that's a good point, there is a strong possibility many fighters have and are currently abusing illegal substances and just not getting caught. So for example, let's just say every UFC fighter currently using TRT used to be on steroids and never got caught, is it fair to give them a pass just because they were smart enough to pass the tests?
The difference is that Mir on TRT has looked terrible (vs. JDS, DC & Barnett). If he was knocking people out left and right, people would be complaining about him the same as Vitor. It's the same reason why Bonds was a pariah but Brady Anderson (Orioles) was forgotten and left alone.