The reason they're shooting it is to *try* to make it go off.
Typically this would be put in a bomb pit somewhere nearby. From reading the comments this is fairly common practice there. This one nailed it pretty well:
"I know many Jewish Israeli people who had their bag shot just because they left it unwatched for a couple of minutes. Yes, this is the unfortunate reality that Israelis live in, where Palestinian terrorist would do anything (such as put bombs cowardly hidden in laptops) to intentionally hurt innocent civilians. These are precaution measures intended to prevent loss of innocent lives (yes, sometimes at the cost of a cherished laptop because of a possibly careless border officer)."