Well, they were expressly motivated by the French government's treatment of those of Islamic religious leanings. Since the French have decided to ban most methods of expressing that affilition.
There was an article on it in Foreign Affairs this month (I happen to have a copy for a paper). The French government has really made a mess of handling their immigration issues, and the riot wasn't enough to wake them up; they went and made a bigger hash of it instead.
The French need a freedom of religion clause somewhere, for their own safety.
The riots were instigated by the deaths of 2 immigrant youths who reportedly died hiding in an electricity station from police (performing ID checks; reportedly it was not unusual for immigrants to be detained for up to 4 hours without any charge for questioning). That was the
expressed motivation for the initial protests that eventually led to the rioting.
However, the general consensus as far as I have seen is that the cause of the riots was long term and historic social inequality for immigrant families living in slum council estates, including perceived police (and state) prejudice, unemployment and discrimination. There has never been a suggestion (I have seen before) - beyond those who gain from making it, such as right wing commentators - that religion was any causal or motivating factor, and the chief of French Intelligence has stated there was no religious motivation to the riots in
Le Monde (unless, of course, you mean discrimination against Muslims - or indeed Africans - in French society).
The French do have a freedom of religion law dating from 1905 that forbids discrimination and seperates church from state; this is the very reason for the famous headress ban. It is also enshrined in the very first part of the
French constitution;
"France shall be an indivisible, secular, democratic and social Republic. It shall ensure the equality of all citizens before the law, without distinction of origin, race or religion. It shall respect all beliefs. It shall be organised on a decentralised basis."