covered T-signs

[lbmango.livejournal.com]

I've been meaning to ask this for weeks, and I havn't seen anyone ask it here, so I'll assume that everyone but me already knows, but why are the T-maps covered in the station? It's in every station I've looked at, but it's only the maps that are on the walls on the other side of the tracks, not the ones that are in the area that people walk...

anyone know?