QUOTE (Walpurgisborn @ Apr 20 2010, 07:41 AM)

Why?
Assuming I'm an evil cranial bombing kinda guy, why bother removing it when the person is finished with the job. Say they do what I tell them to do, fine and dandy, why not tell them they have to do something else.
Corp really has no incentive to remove the cranial bomb. So why would they?
It has to do with the kind of story you are trying to tell.
In a realistic world, if someone threatens you via violence, blackmail, etc., then the smart thing to do is to not play by their rules. As you said, there's no reason to assume they will let you off the hook just because they got what they want. Find a way to turn the tables on them; come clean about the subject of the blackmail and take him down with you. Shoot the hostage. Do whatever you can to remove the opposition's power as fast as possible; negotiation is at best a stalling tactic.
However, Shadowrun plays by a slightly different set of rules - it is generally a dramatic narrative. Action Movie, Caper Flick, Spy Film - the archtypical Shadowrun plotlines are not based on reality, but on cinema. If a guy tries to kill you, you don't call the police, you engage in a cat-and-mouse game of escalating destruction across international borders with car chases and witty banter, culmulating in a mano-a-mano duel, preferably at midnight on top of a building in the rain.
Dramatic narratives are perhaps less realistic, but they are a hell of a lot more fun. A willingness to play along with certain conventions will encourage this style of gameplay. In this case, the
leverage that the bad guy has is designed to get you to 'play along' for the duration of the story. This same "adventure against your will" story can be told with a dozen different kinds of leverage - cranial bombs, kidnapped family or loved ones, doomsday devices, curses, geasa, snipers, bombs that will kill random innocents, etc. If the implicit challenge of the story is "survive the suicide mission", then an escape route will present itself once that challenge is overcome. If the implicit challenge is "get out of this mess", then the suicide mission is the less interesting part of the story and the character is expected to focus on the person who is holding the detonator instead. Figuring out which way you are expected to go is the trick.
Consider "Escape from LA" vs the "Die Hard" movies. In "Escape", our anti-hero is unwilling to take action to save anyone but himself - the 'good guys' set him up with a timebomb to get him to take out the 'bad guys'. If he hadn't played along, they would have simply executed him, no movie. That would be boring. Conversely, all of the "Die Hard" movies are based around a villian who is counting on the 'good guys' believing his story and playing along - the hero spends the film sabotaging the bad guy's plans out of sheer cussedness, and eventually is shown to be right.
The bottom line is, the Cranial Bomb can be a way for the GM to lead you into an awesome adventure - or getting rid of it can be the adventure. Figure out which one the GM is planning on and you will have more fun with it.