You cannot help it if someone mixed coins before they send it to you... so how can they penalize you for that? It is something different if you mix the coins before you deposit it onto your account with a regulated exchange.
I think it will be unfair to the user that just received btc from someone who did mixed his coins. And also, mixed coins are not only because someone is hiding illegal activity, there are other reasons why he opted not to trace the btc from somewhere else. Like for example in gambling, there are exchanges which do not accept if it is from gambling sites even though it is totally legal to some countries. So to avoid flagging their account, they just use mixer services to avoid potential loss of their funds. I believe exchanges should weigh things before they can implement such tight rule over allegedly mixed coins.