"Free to use bitcoin for any purpose." That could be an oversimplification. Some would object to bitcoin being used for money laundering, terrorism or crime. The idea and spirit behind bitcoin isn't to create a currency that can be used for literally anything whether it be moral or immoral.
It doesn't make any difference. Bitcoin can be used for anything including illegal. It is designed as decentralized. It just so happen that people have taken advantage of the "flawed" of bitcoin for illegal activities. But you can't blame bitcoin for that. Anything can be used for illegal activities not only bitcoin.