I can answer this one. It was actually a bug, not an intended feature, caused by our transitioning to Template Toolkit over BML. (Apparently TT doesn't like some of the variable names that were used in the BML version, but this wasn't realised when the logout page site skin was rewritten using TT.)

The bug's been fixed as of today and should go live in the next code push. (Not sure when that'll be, but I think it'll be soon.)

[edit: Oops, referred to the wrong thing. It was the site skin I meant, not the logout page.]