Respect for the office.
They are our head of state and head of government. Regardless of the man that holds the title, it's a sign of respect that we take care of those that held that job.
Obama is an ass of the highest order, but if I met him I would still show him the respect that the office affords him. It's a matter of living in a civil society.
They are also given protection for life because they do know a lot of secrets. I don't need to know where all the bodies are buried, and I don't really want the rest of the world to find out either.