> Is there some sort of contract somewhere that says they won't edit messages and will maintain an audit trail?

Legal contract? No. Social contract? Yes.

> Otherwise, I think they are free and clear to do whatever they want to do with any subreddits, posts, or comments.

I'm surprised this is where you went with your rationale. The parent was clearly saying that the intent of the communities is the ability for people to freely speak each other. Any function, especially by authority, to undermine that will erode confidence and thus eventually cause people to use the site less.