How is this different from transferring the coins from an unspent block to a brand new address?
Assuming the anonymity provided by a mined block has some value to the buyer, the miner could just transfer 50 BTC from the unspent block to the buyer's address and receive 60 "dirty" BTC from the buyer.
They just have to be careful that the coins don't get mixed.
From a new block, there wouldn't be a history. Nobody would know whom to ask where he got his BTC.
Compare to BTC acquired from a reputable exchanger. It might be possible to tell what exchanger the BTC came from based on other transactions it was combined with. Someone goes and gives their real name to a KYC-following BTC exchanger thinking they are buying "anonymous money", and then they go do something terrible with it thinking there's no link to the BTC purchase, but there is. The BTC he bought may very well have enough traceable history that would lead to the place he bought it, there is just no way to know for sure.
At least if he got BTC from a new block, that risk would be greatly lessened, because one could be sure there was no public information available about where the BTC came from.