The main business of the media is to sell audiences to advertisers. Advertisers thus acquire a kind of de facto licencing authority, since without their support the media would cease to be economically viable. And it is affluent audiences that get advertisers interested. As Chomsky and Herman put it, "The idea that the drive for large audiences makes the mass media 'democratic' thus suffers from the initial weakness that its political analogue is a voting system weighted by income!" [Ibid., p.16]
Political discrimination is therefore structured into advertising allocations by the emphasis on people with money to buy. In addition, "many companies will always refuse to do business with ideological enemies and those whom they perceive as damaging their interests." Thus overt discrimination adds to the force of the "voting system weighted by income." Accordingly, large corporate advertisers almost never sponsor programs that contain serious criticisms of corporate activities, such as negative ecological impacts, the workings of the military-industrial complex, or corporate support of and benefits from Third World dictatorships. More generally, advertisers will want "to avoid programs with serious complexities and disturbing controversies that interfere with the 'buying mood.'" [Ibid., p. 18]
This also has had the effect of placing working class and radical papers at a serious disadvantage. Without access to advertising revenue, even the most popular paper will fold or price itself out of the market. Chomsky and Herman cite the UK pro-labour and pro-union Daily Herald as an example of this process. The Daily Herald had almost double the readership of The Times, the Financial Times and _The Guardian combined, but even with 8.1% of the national circulation it got 3.5% of net advertising revenue and so could not survive on the "free market".
As Herman and Chomsky note, a "mass movement without any major media support, and subject to a great deal of active press hostility, suffers a serious disability, and struggles against grave odds." [Ibid., pp. 15-16] With the folding of the Daily Herald, the labour movement lost its voice in the mainstream media.
Thus advertising is an effective filter for new choice (and, indeed, survival in the market).