OK, but those coins don't actually belong to the Government though. They belong to the users of Silk Road. If I bought some drugs and gave the dealer $5 and was caught, that $5 note as well as the wallet in my pocket that contains a few more dollar bills, some credit cards, my drivers license, etc. is still mine, is it not?
Also not all of the trading that was going on was illegal:
There were also legal goods and services for sale, such as apparel, art, books, cigarettes, erotica, jewellery, and writing services.
Link:
http://en.wikipedia.org/wiki/Silk_Road_%28marketplace%29