I may be wrong, but I believe it's so the suicide bomber can't back out at the last minute. They have a spotter that makes sure they're in position then blows them up.
I am, however, with Fox News on disliking the terminology "suicide bomber" just because it puts the emphasis on the death of the murderer rather than the innocent folks whom he murders. The suicide aspect is really the least important aspect of the act.
I wish things were that simple. There are several variations that makes this logic less convincing, from social pressure ('all families must make sacrifices') to outright catch-22 ('if you do not do it, we will kill you and your family').
Especially in the latter case, I wouldn't call this suicide.