Why do major corporations decide to underwrite programs on Public Broadcasting Service? In conversations over the past 10 years with many representatives of corporate funders, self-interest has never been given as the main reason. Instead, the first rationale is always public service - the fact that PBS offers viewers high-level alternative broadcasting which the corporation in question believes needs to be made available to the great American public.
Only after that objective is stated will anybody admit that there are also many image-building advantages to underwriting PBS programming. And these are undoubtedly valid, too.
When a company is having image problems - strikes, lawsuits, trouble with regulatory agencies - that is often the time when somehow the public-service aim is made more concrete with actual funds forthcoming. Sometimes a company will be in the process of changing its name, or acquiring other companies which complicate its identity. Sometimes a company will be changing its product or its direction and wishes to establish its new identity.
In such instances, PBS is a comparatively inexpensive way to reach an audience - almost subliminally.
I talked to several current PBS underwriters and asked them two basic questions: What do American TV audiences gain from your funding? And what does your company gain? Here are some of the answers:
* For AT&T, underwriter of ''The MacNeil/Lehrer NewsHour'' (Mary Delle Stelzer, division manager of corporate underwriting): ''The American public - including our customers, shareowners, employees, and future employees - care very much about what's happening in the world. 'The MacNeil/Lehrer Report' was the best news analysis program on TV, so we underwrote it. When the idea for a full hour came up, we felt the same quality in a full hour was something we should bring to the American people.
''We think the PBS audience is a key audience of people who are concerned, willing to take action in their local communities. The kind of people important to us. By underwriting, we get the opportunity every evening to equate top-quality news with a top-quality company. That helps us in terms of our corporate image. The old Bell System was clearly seen as a quality company. Our underwriting the show extends and enhances that reputation and makes it clear that the new AT&T has that same commitment.''
Ms. Stelzer responds to criticism that the program provides AT&T with back-door access to news programming this way: ''If we could control MacNeil/Lehrer, the show wouldn't be of the quality we would want to be associated with.''
* For J. C. Penney, underwriter of ''In Performance at the White House'' and ''Working Women'' (Harvey McCormick, manager of public relations): ''Penney is in the midst of trying to broaden its appeal in terms of getting more contemporary men and women to shop in our stores. These people would also be interested in the type of programming to be found on PBS.''
* For the James S. McDonnell Foundation, underwriter of ''Smithsonian World'' (Michael Witunski, president of James S. McDonnell Foundation and vice-president of McDonnell Douglas Corporation): ''Our foundation addresses itself to fundamental issues in American society - the development of the individual, things which come under the heading of mental development. We fund the Center for Study of Higher Brain Function, for example, and the UN Association, U.S.A. The Smithsonian is the custodian of the artifacts of the emerging American civilization, and we feel the series on PBS is very much in line with our aims.
''Although it is the James S. McDonnell Foundation, not the McDonnell Douglas Corporation, which is the funder, the corporation will promote the PBS show in its advertising.''
The James S. McDonnell Foundation is named for the founder of McDonnell Douglas.
* For the Keebler Company, co-underwriter of ''The Making of a Continent'' (Dave Mishur, public relations manager): ''We feel that PBS needs the support of major corporations. Since we are concerned about kids and our corporate philosophy stresses healthy ypoung people in mind and body, our funding of 'The Making of a Continent' is an offshoot of that philosophy. We are also funding teachers's guides and classroom posters. Of course, we are interested in making the name of Keebler better known, but that is not a major factor in the funding of the show.''
* For Johnson & Johnson, sole corporate underwriter of ''Nova'' (Marshall Molloy, manager of editorial service in Corporate Public Relations Department): ''We believe 'Nova' is the most revered science show in all of TV. And thus it is a particularly good way to reach young minds interested in science and medicine . . . of intense educational interest these days. When we fund 'Nova' we gain the knowledge that we are helping to support an excellent scientific effort, and we get a measure of public recognition that we are doing this. Our employees are proud to work for a company that underwrites this kind of programming. It brings an extra measure of respect and recognition for the company.''
* For the Sperry Corporation, underwriter of ''Nature'' (Del Kennedy, director of corporate advertising): ''PBS offers a type of quality programming not available on the commercial networks. It deserves to be made available to the public. Sperry, as a high-tech company, wants to be associated with high-tech state-of-the-art programming - and 'Nature' certainly is that and the best in the natural science field. We want an association in the public's mind with quality. It's a subliminal thing.''
* For the Ford Motor Company, underwriter of ''Washington Week in Review'' and ''The Oil Kingdoms'' (William Sheehan, executive director of corporate public relations): ''Image building is a form of public relations. We choose to fund programs which are worthwhile. 'Washington Week' is certainly one of the best programs in the information area, and funding it puts us in bed with the good guys. It's just as simple as that. We support all kinds of worthwhile things to create an atmosphere in which people think well of us. It's just as simple as that."