The simplest answer is that returning it isn't free.

Also there is a good question is there some limit or ratio of tainted coins that would be considered non incriminating. Or if there isn't would single satoshi taint all coins? If there were wouldn't dilution of funds be possible? That is wash them by sending to wallets with enough funds...

Ok so use it to pay the fee, returning the rest.

You just washed part of the BTCs which is now owned by the miner.

That's assuming a BTC miner even agrees to process the transaction. Why would a BTC miner want to deal with the headache of tainted bitcoins? It's such a problem that Marathon Digital Holdings, a major bitcoin miner, has stated they will refuse to process transactions from tainted addresses. In the near future, I expect more mining pools to do the same.

If they were send to you, someone does process them.

And why would currency users even bother with it? If I receive a payment for something, I don't feel the need to pretend to be the police.

Why would you give money to a criminal?

Would you return stolen goods to the thief? How about return it to the owner/authorities. May not be simple but certainly better than sending it back. If all else fail there are black hole addresses to forever lock the BTCs.

Every so often a moron shows up trying to sell the idea of colored coins... it is almost as if they've never heard of "fungibility".