Oddly enough, if terrorists start using cryptocurrencies more, it might actually be easier to track them, though freezing funds would be a challenge. Although Bitcoin is anonymous in the sense that you don't have to give your private information to anyone just to be able to use it, all transactions are publicly recorded on the block chain and leave a trail. People who know what to look for are thus able to track cryptos as they move from address to address. They can then connect those addresses to their owners by looking at other online behavior. I don't know how it all works, exactly, but I have read more than one article explaining how using cryptocurrencies actually leaves more of a digital trail than people realize.