I don't know enough about the route they were taking and who uses the roads they were on to really venture a guess. But yeah, hitchhiking in area where militants/terrorists can even get to seems insane.
Here, I guess, is what we really don't know or understand. Did the hitchhiking even have anything to do with it? I mean, if terrorists can even use the road they are on (ie, a public road both Israeli's and Palestinians regularly used), all it would take is someone pulling up, aiming a gun at them, and demanding they get in. In that case this could have happened to any Israeli's who happened to be along side the road - wrong place, wrong time. The other possibility is they were fooled into accepting a ride which might indicate killers who could pass easily as either Israeli's or Palestinian business professionals or something that seemed safe.
Something else to consider here is that -- if the article I read was correct -- this abduction took place in an area of the West Bank that's supposedly controlled by Israel, not the Palestinian Authority.