They probably own them for a lot of reasons: WaPo was failing and running a loss at the time that Bezos purchased it. Twitter is in the same boat due to poor management; it's losing money and headed towards failure. If Musk can turn it around he can A) Make a bunch of money, B) Save a useful tool for online communications and C) Promote his values when hiring leaders at the company.

I doubt Elon is going to be personally moderating every Tweet, however it is likely that their corporate values system might change.