Yes, sure. It's because ISIS controls the territory that it has in Syria and Iraq much like a sovereign state and is the primary authority.
It's able essentially to clamp down and make the economic rules of the game, and it taxes all kinds of economic activity within the territory, from water to electricity to government salaries, which are still being paid by Baghdad to government employees who are living in northern Iraq, to goods and services. Trucks that are driving through its territory are taxed at checkpoints. This is a very lucrative way of self-funding that ISIS has used, all the way back to when it was known as al Qaeda in Iraq.