Everything in this world have pros and cons, this one is the biggest disadvantage of bitcoin that it is being used by criminals to do illegal activities like money laundering and trade of illegal stuffs. A big hurdle for government is to restrict these criminals and arrest them because transactions are anonymous and it is difficult to locate the criminals from it. BTW after regulation of bitcoin and taking additional measures it is expected that these things can be controlled in future.
And it seems like they have a hard time doing it.
The only one's who are getting arrested are those website wallet owners or trading website companies.
It needs help from the bitcoin holders itself. We are the ones who are being offered to sell our coins in a much better price just so they could do their illegal stuff.