If they did it voluntarily, it's still "brainwashing" - because they believe the silly motives of radical Islam and think that they might have any influence in this deadly war.
People tilt at windmills for all sorts of reasons. To say that voluntarily taking extreme and quixotic actions in the service of extreme and quixotic beliefs is itself brainwashing simply because of the extremity and quixotic nature of the actions and beliefs strikes me as lazy, and something of an insult to people who actually have been brainwashed. It doesn't help give us any understanding of how these things happen or how these people think.
Yes I agree, brainwashing demand much more than this. These girls choose to become extremists, yes they may be young and stupid, but so are most of the people when they start embracing extreme ideologies.
So while I feel sorry for the parents, whatever happens to these girls, they have brought it on themselves, and I feel no more sorry for them, than for any other person, who tries to spread pain and suffering to the innocent.