Every instance can impose site-wide rules (e.g. beehaw disabled the downvote button, I believe, so you can’t downvote anyone in all of the communities that belong to the beehaw instance).

That’s true for those on beehaw. But if I look at a thread hosted on beehaw from another instance I can downvote. Since I’m looking at a copy/synced version of the thread and I don’t have downvotes disabled.

Still though. It limits the abuse of downvotes to an extent.

Here’s an example screenshot. Note the downvotes are there.

Example 2 from beehaw