Well that's part of the point.

You are not the only one laundering the coins through that service. Many many many people are. It just so happens that one of those addresses paid for drugs. And there really should be no way for them to link that back to you. It's part of a laundering service that so many people use is and are connected to it.

Silk Road would have to have a leak that meant having the drugs shipped to you was the issue. In that case it wouldn't matter at all what you did with your coins, how you paid for them, nothing. The issue there would only be that you had drugs shipped do you. In this case the only leak that needs to happen is with Silk Road. But I just don't see that happening. I could be wrong though.

Unless you mean other people in that laundering network are paying for illegal things and you are not or whatever. In that case I go back to the fact that too many people are using that same laundering service (or should be..) and they would not be able to track that one address back to you... And once again, that is part of it. They wont pin the purchase on you but they also wont pin it on the other guy or the 100s of other people who used that service.