The major public service television network in the United States is PBS (the public broadcasting service). The main source of funding for PBS is the Corporation for Public Broadcasting, a nonprofit corporation created by Congress in 1969.

Section 396 of Title 47 of the United States Code says:

• (1)it is in the public interest to encourage the growth and development of public radio and television broadcasting, including the use of such media for instructional, educational and cultural purposes;
• (3)expansion and development of public telecommunications and of diversity of its programming depend on freedom, imagination, and initiative on both local and national levels;
• (5)it furthers the general welfare to encourage public telecommunications services which will be responsive to the interests of people both in particular localities and throughout the United States, which will constitute an expression of diversity and excellence, and which will constitute a source of alternatice telecommunications services for all the citizens of the Nation...

sources:

• Corporation for Public Broadcasting: http://www.cpb.org
  
• US Code Title 47 Section 396: http://www.law.cornell.edu/uscode/47/396.html