Non sequitur. You fail at analogies. The proposition that some criminals may abuse a good service, just as criminals abuse the Internet itself (thanks, o_e_l_e_o), has no logical similarity to the proposition that a service actively cheating its own users is a scam.
The base of his argument was that it was an opinion.
You believe YoBit is a scam because their coin won't have value and tag people who wear their signatures.
If someone thinks mixers exist to launder money for scammers he's righteous to red tag people wearing their signatures.
You can disagree the same as we do now.
If you can tag based on your opinion, why can't everyone?
Trade isn't an opinion.
You either scam someone, or you don't.