Contaminated bitcoin is not a rare case. This happens very often. And of course, we always have a solution for that, so many systems can handle your concerns. Right on this forum, bitmixers and bitblenders are the two most reliable mixing systems. Try it, and you will get the best results. Don't hesitate!
In fact, many tainted coins were spread to others by the "mixers". I guess we do not need to worry about this for the time being. The exchanges (includeing bitcoin <--> Altcoin exchanges, such as Poloniex) are doing their best to avoid illegal activities.
The "mixer" is not a means of spreading contaminated currency; otherwise, it will clean and secure your currency, which is completely safe and legal. Therefore, there is no reason to talk about polonox avoiding illegal activities, which is irrelevant. But in reality, we can not distinguish between contaminated currencies, so the mixer is just a precautionary measure.