> Users were told that upvoting rule-breaking posts would risk a sitewide ban
The consequences are very gradual, starting with warnings, then going up to temporary and eventually permanent ban. So it won't come out of nowhere, you will have plenty of change to adjust your behavior.
> Reddit gets to declare any content "bad" by arbitrary or overly broad or even unspecified rules. That's the uproar.
Not really, they have very precise rules and content policy, which they use to take down content.
I do agree that being told which content you're being punished for, and what specific rules said content broke, would help users adjust their behavior, but you also run the chance, as with any sort of moderation, to teaching abusers ways to bypass the system. That's why things like shadowbanning exists.
At the end of the day, as yesterday's ruling for Youtube showed [0], these websites are not public forums, and they are free to have any rules they want. If you're not happy or you don't trust that they apply their content policy fairly, you can move to a different website with different rules.
[0] https://www.bbc.com/news/technology-51658341