Yes, I'd absolutely add such sites that just arbitrarily reject certain 'coins'.
Anyone trying to come up with a definition of 'pollution' of BTC is actually attacking Bitcoin's fungibility and should be called out. That's what this thread is for.

An easy way to understand it is the comparison to 'dirty fiat' that LoyceV has brought up: nobody rejects a paper bill you got by winning at a casino or a lottery or one you got by selling drugs. Nobody inspects bank notes for illegal substances or checks the serial numbers.

Another point to consider would be that laws differ around the world. I may live in a state where marijuana is legal, so selling it for BTC would be absolutely legal; however if I send it to someone in a different state, and they'd somehow label it as 'drug money', it couldn't be spent there. Wouldn't that be pretty crazy?
If this seems far fetched, think about something else: booze. In most parts of the world, nobody would label me a criminal if I were to sell home-brewed beer. But there are places where alcohol is indeed illegal.

What I'm trying to say is: we can't restrict Bitcoin that was used for activity X because it is illegal in state Y.