Why do Presidents and their families get protection...

Question

...by the secret service even after the President in question is out of office and is a "private citizen" again? Shouldn't their safety be up to them at that point? They're usually not contributing anymore to the country, so why should we contribute to their safety?

Answer 1

Because they are still much more of a potential target than ordinary citizens.

Private protection would not be enough. They sometimes need access to intelligence, and that knowledge cannot be handed to private security agencies.