According to Larry Lessig,

"CBS is said to have refused to run MoveOn’s winning ad, citing its policy not to run commercials dealing “with controversial issues of public importance.” CBS will instead run ads from the White House Office of National Drug Control Policy — apparently an issue of public importance that is not controversial. Who would of thought an ad criticizing a $1 trillion deficit was more “controversial” than an ad about the war on drugs?

Here’s what the true libertarians have been saying for a long time — the biggest reason to worry about concentrated media in a world where media is regulated is exactly this."