Crime can be funded with anything: from cash,gold,silver to bitcoin. So all these methods can be hardly traced,not only bitcoin. I don't see any major reason why bitcoin price would decrease if it is used in crimes. Most surely us dollars are used in crimes daily and the price doesn't fall right? 
There is no solid proof with anyone saying that bitcoin is used for crimes but all of them are just the assumptions that the people randomly put in everything. There is a possibility that it can be used but we cannot say that bitcoin is responsible for the crimes because criminals and crimes were present before the invention of bitcoin.