The cynical part of me is in complete agreement. I hate that I assume ulterior motive to charity, but Bloomberg wants to be in politics, and this is a big move at the very beginning of the primary races.
Its a non-zero sum game, why worry about such binary events of completely charitable or not.
Trump has set the precedent that this is not necessary anymore.
Now for the political science: If you want to win brownie points with both the wage slaves and the “orange man bad” crowd that would be elated to see everything the opposite of Trump, you can jubilantly release your tax returns if they look good or relatable.