"Extortion and illicit taxation systems are also a significant source of income for ISIS, and potentially one of the most sustainable. Prior to capturing Mosul, ISIS was already earning $12 million a month in the city alone. This is now being replicated, though in a more organized manner, across ISIS-controlled territory and covertly in other areas under its partial influence. However, it should be recognized that this ‘extortion’ and taxation is not always done unilaterally and solely in ISIS self-interest. For example, a sophisticated ISIS taxation system on the main highway between Jordan and Baghdad has been developed which replaces the government’s import tax by charging reduced rates for the transport of goods into the Iraqi capital. The trucking business across western Iraq is primarily controlled by Sunni tribes, and therefore, by imposing lower taxes ISIS earns a steady income but offers its tribal guarantors an opportunity to increase their earnings. Similar systems are in place elsewhere in western Iraq and eastern Syria, with an overriding emphasis place upon this dual focus of earning money while retaining a ‘buy-in’ from tribes that ISIS existentially relies upon for its societal survivability.
so without troops on the ground to attack ISIS and to develop a sense of security so that nobody thinks that they have to pay to ISIS to be sure that their livehood and business will not have any risk of being killed, ISIS will be sure to have at one side enough resources to pay its combatants and at the other side have secured the continued commerce in the territories that she controls so that 'everything looks normal' and people can 'get on with their lives' (they think)
and neither the Iraq army nor the Iraq leadership are showing any real willingness to go to a real war with ISIS because they are holding back (and the US and the socalled alliance is NOT giving them the military hardware and operations they need to make a real difference)