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1. the owner of blacklisted cowns can change them into not listed coins within minutes
You can declare all outputs illegal which had part of the coins origin from that address.Quote
2. the blacklist must be accepted legally worldwide . otherwise the listed coins can be spend for services which do not care for the list
The us can impose FATCA onto the rest of the world, why should they not be able impose any other law to the rest of the world. If a major part of economics would happen in Bitcoin and they would enforce that their companies/citizens must only accept clean coins, then you would have two prices on MtGox (for clean and unclean coins).