So you accept Bitcoins from someone, and because their inability to choose the inputs for the transaction they send to you, you end up with some coins that were "unclean" a few transactions ago. Or maybe you don't realize this, because you don't happen to subscribe to the arbitrary group watchdog that monitors this shit - until you go to spend those coins and someone who is subscribed calls you a scammer.
The fact that you have unclean coins does not mean that you are a scammer, it means that the scammer is somewhere between you and the victim. If you end up with unclean coins, the website will allow you to get in touch with the victim of the scam/theft.