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? 
I think it's normal when bitcoin is used as crime funded or other illegal activities. Because bitcoin is also money that has a price. And not just bitcoins used for illegal activities but as you say, cash, gold, silver are also used as tools. This is a public secret