How can you know that? if ever there was a survey that happened. Because I do not think anyone knows for sure criminals, more use dollars or bitcoin. But I think because the dollar is better known, the majority of criminal use this currency. IMO
But I agree if the currency can not be considered criminal. Because currency only objects used by criminals and money should not be blamed.
Criminals and police could use dollars or another currency as they want, for something bad or good, same thing as bitcoin. But bitcoin has advantage to used by criminals due to it's anonymous and fast transaction.