It's kind of similar to tax evasion really. Most people that got their money through illegal means wouldn't file the tax properly and thus need to hide it. it's just an easy way for the government to collect the taxes due to shut down the entire system.

If you noticed in the press coverage that Preet Bharara invites you to get your money back if you were using Liberty Reserve for legit purposes. Well criminals aren't obviously going to call. and if you call, well, probably need to start reporting them in taxes as well. govt takes all.

http://www.nytimes.com/2013/05/29/nyregion/liberty-reserve-operators-accused-of-money-laundering.html?pagewanted=all&_r=0