It is a madness within Islam. The Copts essentially welcomed the Arabian invaders, as they were oppressed by the Byzantines. The 'conquering' of Egypt was a major coup and hurt Byzantium greatly. For a good while, the Copts prospered through lower taxation and freedom to practice their version of Christianity.
Note 641 CE entry:
http://www.pbs.org/wgbh/pages/frontline/teach/muslims/timeline.html
So, if the earliest Islamists did not have open warfare on the Copts, what is the basis for these latter day Islamists to do so? Why do they have the correct view? The same questions apply to Roman and earlier era pre-Islam artifacts and sites. These were present in Muhammad's day. Muhammad didn't destroy these. Was he not fully a Moslem? Did he not fully comply with Islam?
Why cannot appropriate Islamic groups denounce these extremists openly, loudly and widely?