Given it's been like this since 1.1, I'm surprised this hasn't been mentioned before. There is an element of expectedness (code-wise), but equally it would be unexpected to the end user. I should note that the various other message element style properties don't take any permissions into account, though nothing is explicitly called out as a privacy entry for those values specifically.
Logistically, I think this will probably end up going down the route of not showing the info at all when you only show it to followers (or no one) and showing it conditionally if you only show it to members only.