Does your service include sites that do not accept bitcoin from gambling, porn, drug promotion, etc.? Or are you talking about mixing/“polluted” coins specifically?
I'd say it's about any reason to consider some Bitcoins more equal than others.
Note that this isn't about exchanges freezing funds owned by drug dealers, that's understandable.
It's about freezing funds from the bar where the drug dealer bought his coffee.Do you have any evidence that this is happening? I don't think it would be difficult for a blockchain analysis company to see the difference between someone buying $100 worth of drinks at a bar, and a someone selling $100,000 worth of coin.
When you spend 1
BTC, you are spending a very specific output of a previous transaction, or a very specific set of outputs from previous transaction(s). If you are spending that 1
BTC, the transaction fee you will need to pay will depend on how many outputs you are spending.