Usher is likely voting for Kamala but doesn't feel the need for people to know that, so I doubt it has anything to do with him not liking or supporting her. Rather, this is his way of saying it's not our business to know. Honestly, whether you're an average citizen or a celebrity like an actor/actress, athlete, artist, etc, you should keep your political beliefs to yourself for your safety.
Unless you're part of a political campaign group or grassroots type of situation, you should keep sensitive matters like this private. There's a reason why we vote in booths or by mail to keep things private and confidential. Outside of talking on this site for dialogue purposes, I don't walk around bragging and flaunting my political beliefs and preferred candidates or asking people about theirs because it's not my business to know, just like it's not theirs to know mine.