While riding the Red Line, I noticed a graphic advertisement aimed at raising awareness of the plight of children in Syria (sorry for the blurry photo).

Below the advetisement was this message:

The advetising space is a designated public forum and does not imply WMATA’s [Washington Metropolitan Area Transit Authority’s] endorsement of any views expressed.

It seemed like a health warning you would see on a Tobacco or Alcohol ad.

Has anyone seen this message before? Do you know if WMATA makes the organization place this notice on the ad? Was there some court case mandating this? If they decline to place the message there, are the advertisements rejected? Is forcing this message on the advertisement compelled speech? Is this a reasonable time, place,and manner regulation?