FIFY.
Admittedly, much of their "revenue" comes from the resources in (and under) said land; however, they bring in a fair amount of money from mafia style "tax" schemes, and even more from Sunni sympathizers.
They need to hold land and establish rule over the population to collect taxes. As for sympathizers, according to
the economist, that money pales in comparison to the revenue gained from smuggling oil.
The way they generate revenue is more akin to drug cartels in South America. You can't cut those guys off with a demand-side approach or attempting to interdict their supply lines, what makes you think it'll work with ISIS?
I think we're losing sight of my point on this issue. Regardless of how one defines a State, or how governed that State might be, the geography in the Levant into which IS could retreat, should they be defeated by direct military action in the urban centers, is massive. It would be very easy for them to revert to insurgent mode. That likelihood complicates many avenues of potential Western military escalation.
Which is again fighting the last war. The goal of a war with ISIS isn't to liberate people from a dictator and show them how great democracy is, it's to eliminate the enemy and install a government that doesn't support global attacks on western societies.
My point is that by calling them a state, we can have a well defined enemy that doesn't start a polarized debate between xenophobes who want to nuke the middle east and SJWs who think Muslims are just a misunderstood and oppressed people. Instead of arguing how peaceful or violent the religion of Islam is, or debating whether to call them jihadists or Islamic extremists, we can focus on waging war on ISIS, who governs a fairly well defined territory.