It's non-copyable because it's added with CSS and not an element on the page. It's done this way because CSS is the only way for a subreddit's moderators to add

It's non-copyable because it's added with CSS and not an element on the page. It's done this way because CSS is the only way for a subreddit's moderators to add a message like this. (Reddit allows moderators to customize CSS, but not otherwise alter pages.)