All retired Presidents have power, at least for a while. They're an inroads to the party structure that, generally, is someone who ensures that your interest will at least be listened to.
There are of course exceptions, like Reagan due to Alzheimer's or Dubya due to pressing the eject seat on politics, but in general.
EDIT: Did you just imply that trying to change the system for the better would get you killed?Believe me, I'd love to be the person who changes everything, but I'm not conceited enough to believe that's real or going to be me if it is. I do what I can, when I can, and that's just all there is. Also, I don't want to die, that's a pretty strong motivation.
Emphasis mine.
It's far from impossible. Every government kills people who they want out of the way, some much more than others, but nobody is ever completely safe. Failing to recognize that is just naive. I doubt it's likely I would be domestically assassinated by the US government, but there's been plenty of people who have gone overseas and never come back.
Even western governments do this from time to time. There was the Earth First lady who got carbombed, the Americans who have been droned while outside the west, the guy who Blair's government in the UK definitely killed and called a suicide. Getting the ire of powerful people isn't the best idea.
And, of course, there's secondaries as well. If the government calls the police to investigate you over and over again while portraying you as a dangerous radical, then they can just hope you get put out of the way. Not necessarily by being killed either, just imprisoned or drained of resources by legal fights. There's also all the people who have been locked out of the US by being placed on the no-fly list after leaving, ensuring that they can practically never return even with citizenship.