The short answer is that intrastate conflict is fought for both greed and grievance. In a 2005 article in International Studies Quarterly, Mark J. Mullenbach defines an intrastate conflict as a period of
military hostilities between government security personnel and members of one or more armed opposition groups within a state lasting ten or more days, regardless of the number of fatalities.
It should be clear from this definition that intrastate conflict is one of the most common forms of armed conflict in the world today, with multiple examples in any given year.
To take one regional example, there have been numerous intrastate conflicts between the national governments of both Turkey and Iraq and the Kurdish populations in those countries throughout the late twentieth and early twenty-first century. The Kurds in Turkey and Iraq, not to mention Iran and Syria, clearly have a grievance. They are the largest ethnic group in a world without a state to call their own and are dispersed as frequently-persecuted minority populations over four nations. Most Kurds, therefore, were fighting because of this grievance.
However, there has always been widespread political corruption within the Kurdish leadership, and there can be no doubt that certain warlords on the Kurdish side were fighting for greed. Since the leading families of Iraqi Kurdistan have managed to attain billionaire status while the region still does not enjoy the status of a country, it appears that those who fought for greed were, in this instance, more successful than those attempting to address a grievance.