Come on now, you turned this into a debate between MMA and boxing by virtue of your initial post and your follow up. Any promotion operating within the US, in states that have regulatory bodies, have testing. The UFC is the major promotion and they test whether or not they are operating within the US.
That said, there are always going to be athletes that attempt to circumvent these tests - boxers or mixed martial artists or other. It happens regularly in the Olympics despite their strict testing protocols.
In Ken's case it was fairly obvious that he was using. It really doesn't affect MMA's image either as he is so far off the radar - he is 2-7 since 2002. It is great how you've brought this up as if it is some kind of game altering event when professional boxers who have been title holders have routinely tested positive over the past few years without so much as a whimper in popular media. It really doesn't matter.
This reminds me of a recent poll on ESPN which asked people which sport had more performance enhancer usage, football or baseball and the vast majority of respondents said baseball. While both have a performance enhancer predicament, it is ridiculous to think that baseball has somehow surpassed football when it comes to usage of performance enhancers...all you have to do is watch a game.