Some are mercs, most are simply the "road team" for the hometown insurgency of their homeland.

AQ's message is clear and effective, essentially: "you can't win at home until you break the support of the US to the region and to your government." So they go to where the west is most vulnerable and attempt to hurt them there.

To me the only logical point to target is the perception within each of these states these men travel from that the U.S. stands between them and achieving better governance at home. Simply killing them when they arrive only motivates more to come; the routes they travel will flow to the paths of least resistance; and to target them in their homelands is to only validate AQ's propaganda as we step in to help some fairly unsavory state leaders to suppress the insurgent segments of their societies in the name of "counterterrorism."

My $.02