The person sharing their unpopular opinion is doing exactly what’s expected of them, so why downvote it when you agree that’s it’s an unpopular opinion? An upvote would be you agreeing that it’s an unpopular opinion. Downvoting it means you’re either an A-hole or you disagree that it’s an unpopular opinion.
Now I leave you with a joke. Silver was walking down the street and bumped into gold. Silver says "Ey, You’. Gold replies “Ey, G!”
We all know that. Everyone knows this. But getting over the csrnal drive to shit on people you disagree with is difficult for the individual, impossible for the crowd. That’s why I take down voted as a sign of success, or deliberately frame things such that the people who would down vote can’t without looking stupid.