Censorship can be exercised either by governments or private individuals. Private censorship is rather less common, but occasionally presents a serious barrier to freedom of speech; for instance, private individuals sometimes attempt to suppress the testimony of witnesses through death threats—threats that would of course be illegal in a free society. Government censorship is pervasive in our present system, encompassing not only the indirect controls over the marketplace of ideas discussed above, but also policies that intervene directly against particular kinds of speech and expression. The broadcast industry, for instance, is highly regulated; the airwaves have been deemed "public property," which can be used only by licensed broadcasters who are required to serve an undefined and undefinable "public interest" (cf. pp. 4.8:29-30). For reasons to be examined in Section 5, these media are typically loath to challenge the statist tenets underlying public policy, although their programs often create a semblance of controversy by means of debates framed in terms of left/right false alternatives. Advertising of some products is highly regulated or restricted, particularly in the electronic media. In some regions print materials deemed pornographic are prohibited, leading to the creation of new black markets, which serve as a magnet for organized crime. Politically correct "speech codes" and "sensitivity codes" impose stringent limits on freedom of speech, not only on state-run campuses, but also at all schools affected by government funding—in other words, virtually all colleges and universities in the United States. Fortunately, freedom of expression continues to thrive on the Internet, at least in this country, although proposals to censor that medium have received serious consideration.