Why do you think you are allowed to use force to prevent me from implementing some scheme on my own client?
Because you can't control who sends you coins. If somebody sends you tainted coins, what are you going to do about it? You can't send them back, since the address they came from may not be under the sender's control (eg, e-wallets), and you can't keep them but refuse to accept them as payment. Can you imagine if that happened with cash? "Sorry, we can't accept this $100 note as payment, you'll have to give us another one. No, you can't have this one back; no, it's not counterfeit, we just think it might have been involved in some form of criminal activity at some point in the past." You'd have some use of force directed at your face if you tried that in real life.
You don't have to accept coins that have been obtained dishonestly, either. That would be a personal choice. Because bad actors could respond, to honest traders who only making transactions with other honest people, with violence does not invalidate the idea.
The problem is that the people who respond with violence are
not bad actors, they are honest people who have had their money taken without compensation based on nothing more than the belief that the money might have been involved in an illegal transaction at some point in its history. Remember,
YOU CANNOT CONTROL WHO SENDS YOU BITCOINS! If an honest person happens to receive "tainted" bitcoins from someone else, there is absolutely
nothing that they or anyone else can do about it, and any attempt to do something about it will simply be punishing innocent users for other people's actions.
Here's another experiment you can try: Open a store and keep a drug kit behind the counter. Drug test every note a customer pays you with, and if the result is positive, refuse to accept the money but don't give it back. See how long you can last without either getting punched in the face or having the cops called on you. I guarantee you'll be out of business one way or another before the day is out.
I encourage the sympathetic readers of this post to stop thinking about bitcoin as "cash" and start thinking in terms of a distributed file system containing a perfect accounting ledger. The idea is much more sophisticated than "cash" and will lead one day to a system that will allow us to keep track of our debts to each other in terms of personal economic value.
So, everyone's bitcoins will have a different value to everyone elses? How exactly are you supposed to set prices with such a scheme? "This product costs 10 BTC, but only 8 BTC if the coins come from a verified Mt Gox account, with a 50% surcharge if the coins ever touched SR, plus 10% if they came from a coin mixing service... etc" Is that pretty much how it's supposed to work? And if so, how does that make any sense? That would cause problems if you tried it in real life, too.
What you're asking is like asking how Mt. Gox can give you 5 USD/BTC one day and 6 USD/BTC the next day. Why are exchange rates set on an open market with bids and asks any different than devaluing dishonest money? If someone you want to trade with wants to charge you more because you have dishonest money, you don't have to trade with them. I'd encourage you to find someone to trade with that will give you the full value you believe your BTC is worth.
No, it's not like that at all, and I'm not sure why you even think that. They are different because the exchange rate assumes that bitcoins (and dollars, for that matter) are fungible, meaning that one bitcoin (or dollar) is just as good as any nother. This is because when you make an offer to buy bitcoins on an exchange, you have absolutely no way of knowing who, out of thousands of other traders, will actually be providing your bitcoins, and so you have no way of knowing whether you'll end up with good bitcoins or "tainted" ones. As I have already pointed out, setting prices will be impossible if "tainted" bitcoins are valued differently from "clean" bitcoins.