So my understanding of the concept of "Terrorisim" is that you use violence against non-military targets to further political goals.
I can only imagine that ISIS's goal here would be to stop Americans from breaking Islamic law (like drawing their prophet).
Given 1 and 2 above I have to say that they REALLY don't understand American psycology if they think threatening a relativly photogenic white girl will do that for them. If, by some chance, they were to actually succeed in her murder that could go a long way towards making it very difficult in America to be publiclly Muslim.*
We have a pretty storied history of taking attacks personally, and instead of being cowed and terrorized, *expletive deleted*ing *expletive deleted*it up. Some have even said we, as a country, overreact to this kind of thing.
*note that I'm not saying we should take it out on all muslims that many are barbarians, I'm saying that it's a not-that-unlikley outcome were muslims murdering Americans in America to become anything more than a VERY infrequent event.