You pretend that they use as their currency a currency that is not theirs? That is not own by them? Then, if yes, where can find they all that bitcoin needed to pay their fighters? Are very big money that needed. And if they will be able to find all the needed amount, what can do their mujahideens with an internet money? How will use it? Then I bet that 80% of them know internet only as a name and have not used it even one time. They know only Allah, the Quran, the fighting, the various weapons and the women. Everything else is  western culture which want to destroy their customs and their culture. As such must be fight. Are very few they who really are the mind of everything there and maybe they may be the same as a normal people in our area. They might know and use even bitcoin. But is impossible that the money used by ISIS in everything be the money which can be used only by them.