Says who? There is no consensus about what "clean" or "dirty" means. There are companies that sell this idea for a profit, and it's in their interest to convince people like you that their services have value. They don't even share their criteria. What if you discover after accepting it that a "clean" coin was used for bad things nobody knew about at the time? You can never be absolutely sure what happened in the past.
The "taint" BS is an attack on Bitcoin's fungibility, which means any Bitcoin user should oppose this as it's a threat to everything Bitcoin stands for. Bitcoin can't exist if some coins are worth less than others.
Start analyzing the history of dollar bills, and you'll find cocaine on almost all of them. Try to refuse them at a bank, and demand clean bills. See what happens.