TL;DR: Bezos thinks the Post is an important institution for our democracy, and he believed he could help it recover financially by taking advantage of the internet.
Buying the Washington Post certainly forced the world around him to see him as political...even if he considers himself not.
You never considered the idea that the richest man in the world may be political?
It'd be more surprising for me to find out that he wasn't political; you don't accrue that kind of cash without playing political games at at least some point in your portfolio -- even if it's just for the opportunity to reduce your tax burdens.
My point is, I don't think he bought the Post to be political or political reasons. I think he bought The Post because he's a rich collector of nice/novel/rare things..and it seemed like a fun hobby. At the time.
If someone wanted to sell me The Post or let's say WSJ for what I consider pocket change, I would most certainly be interested. Good journalism is something that is going away, but, the obscenely rich can curate and prop up.