The message is displayed on some of the other sites, they just don't use the banner. Also no notice was given to mods of other sites of placing a banner up.
(because that's not what the banner is supposed to be used for nevertheless we will call this exception #2).
They are displayed instead as Community Promotion Ads. This is a method for the community to control what gets promoted to visitors on the site.
I am not sure about the other sites. These ads were designed by badp.
Maybe we can ask him kindly to make ads (or the source image file) for the rest of sites if he is not too busy.