Why is it that I can't downvote some posts?

The downvote system is designed as a way to weed out low effort or useless threads. People use it for disagreements sometimes, but that is not why it was created. Basically, downvotes are not supposed to say "I don't like this", but rather "this shouldn't even exist on the boards". Like if someone is just raging about something, you downvote them as they are not actually having a discussion and you may not want to see someone just throwing a tantrum.

The gameplay+ section only has posts approved by moderators first, so every single thread there does belong there. So downvotes now serve no purpose there.

It's the Boards trying to convince you to try to upvote some posts from time to time instead of just downvoting posts.

You can’t downvote some posts because the content is irrefutable and you are not allowed a disagreeing opinion on it.

Hope this helped.