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.
Why would I want to do that? I received the money in a legitimate transaction, presumably in exchange for something else that I no longer have. Why would I turn around and give it to someone else? Exchanges of currency in real life don't work like that. Odds are that one of the bills in my wallet was stolen from someone at some point, but that doesn't mean that, if I could track down the original person it was stolen from, I should give it to that person. I received these bills from bank, which gave them to me because my job deposits money into said bank. I don't owe them to anyone else. Likewise for Bitcoins.