Bloomberg has also pledged $100 million to Biden's campaign and ran as a Democrat in the primary.
Look, I'm not a fan of Bloomberg, and I'm rather disgusted by his attitude that enough money could win him the election. But I also think what he's doing here is honest. He's paying people's fines to give them back their right to vote, which Bloomberg (and many others) think shouldn't have lost in the first place. So it's at least partially a matter of principle; a matter of fighting voter suppression.
No doubt he hopes that many of these people will vote Democrat, but he has no guarantee, nor even a promise, that they will. I'm pretty sure many of them won't vote at all, and many others will vote Republican. Maybe more of them will vote Democrat, but most of those 31,000 will not.