In Danganronpa V3, voting is mandatory, but voting for the culprit isn't. Typically you can vote for whoever you want and it won't matter because everyone else will vote correctly. This is the only instance where you need to vote for the culprit.
I don't think I've seen this mentioned anywhere, so just wanted to get it out there for people that didn't know.
My guess is because of the lines post-trial where that person says everyone got it right, despite Monokuma typically saying the same thing in other trials regardless of who you voted for.
