[1.] When the government opens up its own property to certain kinds of private speech or speakers, the law treats this program as a so-called “limited public forum,” or sometimes a “nonpublic forum.” One classic example is when a university provides funds to a wide range of student newspapers, see Rosenberger v. Rector (1995). Another is when a city sells advertising on city buses, see Lehman v. City of Shaker Heights (1974). In either a limited public forum or a nonpublic forum, the government may restrict the cont
Source: D.C. Allowed to Exclude Religious Ads from Buses – Volokh Conspiracy : Reason.com