The tactics used by ISIS are quite barbaric. But so are the tactics used by the US. The two are not equivalent, but I'd hope that the side with millionfold more resources would hold itself to a slightly higher standard.
What criteria would you impose upon a grassroots political movement in the middle east? Is it allowed to use force? Should it invest in smart bombs before proceeding to get involved in conflict?
There is a tremendous asymmetry of force between the US and ISIS. Both sides use propaganda, both sides try to scare and intimidate civilians into supporting their cause.
To claim that a suicide bombing of civilians is morally worse than a drone attack on civilians is to be completely deluded. That one side can afford to send in drones that are controlled by pilots thousands of miles away does not matter to the victims of the attack, nor do the details of the decisions that led a suicide bomber to volunteer.
The point is, attacks on civilians are always intended to break the will of the civilians to support their side... the goal is for people to be drifting off to sleep at night and to be startled and fearful at every noise, lest it is an attack (suicide bomber or drone).
It's completely unreasonable to expect ISIS to hold itself to first world standards (drones, smart bombs) when fighting in such an asymmetrical battle. The US itself throws out its moral standards when victory is uncertain.
So unless you think ISIS ought to just surrender and give up, there isn't really any other way for asymmetrical conflict to occur without one side using tactics that the other side's population deems morally reprehensible.
The absence of this obvious realization comes from the propaganda nature of all the news about ISIS. We are supposed to oppose ISIS because ISIS opposes US intervention in the region, and ISIS poses a threat to the US hawkish vision for how it wants to reshape the middle east.
As an American, I'm not sold on the idea that the US should be meddling in the middle east, and so I choose not to buy into the propaganda about ISIS. I do not support ISIS, but I think that all political organizations start out looking like groups of thugs and gradually become less violent and help with the basic needs of their constituents.