Its not weak sourcing. Well it is in one sense (for serious historians) and it isn't (for most Sunni Muslim scholars). This article sources its material largely from the biography of the prophet. The biography was originally written by Ibn Ishaq but that was about 200 years after mohammed had died. He collected oral traditions and wrote the book. 200 years is a long time (especially in that age) and oral traditions are not very reliable. This is made worse by the fact that the Ali Vs Muawiya division was ongoing during that time and so the contents of the biography (what muslims will consider to be what the prophet really said or did) will be of huge political significance in the battle over power.
Even worse, the biography written by Ibn Ishaq did not survive. All we have of it is 2 edited versions of it from his students, ibn hisham and al tabari. So in short, our best biography of the prophet's life is one that was written a long time after his death, it relies on an oral tradition, it was written in a very political environment, and we only have an edited version of it. So you can see why historians can't really take it as credible but its the best thing we have. The same kind of problems actually pertain in the hadiths as well of both Bukhari and Muslim. From a historicity perspective, we have no good source for knowing almost anything about mohammed for certain. There is no example of the prophet as far as historicity is concerned. If muslims take the historicity perspective Islam will have nothing to go by other than the mysterious Quran and it would be impossible to figure out the context and meaning of verses without relying on the hadiths and biography.
On the other hand, Traditional Sunni Islamic scholarship largely trusted these sources (the biography and the hadith books) because they didnt have our contemporary standards of historical evidence. They had their own methods of determining credibility of narrations within these books but that is a field on its own so I wont get into. In this sense the biography and the hadith books are taken as very credible sources for figuring out what mohammed said, did and commanded. After all we have nothing else to go by. The so called "example of the prophet" is derived from these books primarily and the Quran secondarily. @<a href="http://www.juventuz.com/member.php?u=17111" target="_blank">Ronn</a> is right there is plenty of things in these books that more than justifies what ISIS is doing. In my opinion ISIS is actually looking to these books for guidance rather than do things and look for a retrospective justification but I can't back that up other than by using anecdotal evidence from my experiences with Islamists in Egypt and Saudi Arabia.
The funny thing is that most sunni muslims (in the ME atleast) will claim to follow these books, without ever reading them. They will claim that their morality is based on these books since these books tell us the divine commands and laws we should follow. They are taught that the Sahih's (the hadith books) are the most correct books on earth after the Quran but never actually read these books to get those instructions. They hear some selected stories from these books in friday sermons and on TV but they never read the books for themselves. If they do read them, they would be shocked by how different the moral values expressed in these books are from their own moral values.
They wrongfully assume without reading the books, that the morality contained within it is the same morality that they themselves believe in. When I was a muslim, I thought Islam ended slavery for example and that slavery was forbidden and I assumed that the biography and the hadiths will say the same thing. When I actually read these books I had a nice shock. I, and almost every ex-muslim that I know, had the same experience of finding out that there is a huge disparity between what I thought is in these books and what is actually in them. The punishment of gays, the role of women, how to deal with apostasy, the prophet's concubine Maria, his marriage to Aisha when she was 6 and consummating at 9, the satanic verses story and many others. I naiively assumed that my own moral system (which i see as correct since its mine) is the same moral system in these books. It was a huge shock and now I am in Canada