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? 
Maybe more people are using cash to do illegal things. Many of them prefer cash because it is easy to carry and there is no record of evidence. Usually they will tell their people. To bring the cash to the destination. Although complicated but it is safer for them