Without wanting to give credence to the conspiracy theorists, it would be something of an elegant solution.
You provide a territory where jihadists can live out their debased sexual and military fantasies. You portray it as a genuine threat to the west. It gets flooded with all the dissatisfied youth of western Islam... then you bomb the shit out of it.
"Oh, we can't win, it will be impossible to dislodge ISIS, they're an ongoing threat" - more jihadists flock there. More bombing.
You show more in the media about the barbaric living conditions and brutal society that's created wherever Islamic fundamentalism is victorious. More and more people are put off, but those few who get their jollies from that sort of thing race to experience it... and get more bombing.
The big terrorist franchises (Alqaeda, Hamas, etc) get inevitably drawn in by such a rich source of recruits and the black market. Western intelligence operatives will be perfectly placed to infiltrate the networks, leading of course to more bombing.
The big problem with fighting terrorists has always been getting them to sit still in one place where the much more powerful western militaries can get to them. Well, ISIS territories are the perfect solution. Persuade every potential terrorist recruit that its their natural home. Leave to accumulate. Bomb it.
The simple truth is, I don't believe any of the democracies opposed to ISIS possess any of the requisite competence or ruthlessness to do it on purpose, but it could quite easily be happening by accident.