It's private because there are essentially two of you, and your vote is the anonymous you.

https://piefed.social/post/205362

"PieFed accounts now have two profiles within them - one used for posting content and another (with no name, profile photo or bio, etc) for voting. PieFed federates content using the main profile most of the time but when sending votes to Mbin and Lemmy it uses the anonymous profile. The anonymous profile cannot be associated with its controlling account ..."