It's not a specific storyline, it's just the status quo in the Bat Universe.

Maybe he donates to various worthy causes here or there, but it doesn't seem to be a big enough donation to affect his rich playboy lifestyle at all, nor does it affect his budget for gadgets. What he never does is campaign for a major tax increase for billionaires. He's never meeting with the "Bernie Sanders" equivalent in Gotham, trying to push through progressive legislation.

The thing is, his character has the perfect excuse. He's really Batman, and Bruce Wayne is just a cover identity. But, to keep up appearances, he has to put on his tux and go to ultra-rich person parties as Bruce Wayne. It would seem suspicious and draw unwanted attention if his cover identity Bruce Wayne started lobbying for higher taxes on the rich, so he gets to use that as an excuse for why that filthy playboy Bruce Wayne never does those things. It's so that Batman can keep going out there and "fighting crime". AFAIK he's never outright said that that's his excuse for not lobbying for progressive causes. But, he has definitely said that he goes to ultra-rich person functions as Bruce Wayne and plays the idle rich guy because he needs to keep his cover story intact. It's mighty convenient, if you ask me.