I'd argue that the legal and the political system of Islam were just two of the many things that they absorbed from the populations they conquered - especially the political system, as the kept many of the political structures intact in the countries they conquered. The laws were not so different from other Near Eastern law codes.
Actually I think the major difference between the Islamic law and the other law codes from the region is that the Islamic law had stricter religious basis, and could not be quite altered on a whim of a ruler. The laws that were NOT mandated in the holy book (but instead propagated by kings/priests/rulers/etc) were more mutable, and more subject to misuse or misinterpretation.
On a side note, you know what I have just realized? I've been arguing that Islam was of great benefit to the Arabs as without it, they either could not have conquered as much as they have, or would not have been able to hold their territories...