I just spoke to a spokesman for CBS who walked me through their policy.

So here it is.

The network -- as opposed to affiliate stations -- runs no issue advocacy ads in cases where the issue is a matter of public debate. However, they will run political candidate ads.

Their policy of running candidate ads is pretty much moot since it seldom pays for a national candidate to spend money blanketing the whole country with an ad. But the spokesman said they will run them.

Then I asked about anti-smoking ads or the anti-drug ads paid for by the Office of National Drug Control Policy. The spokesman told me that the network does sometimes run these ads and does so in cases where the issue is not one of public controversy.

So, for instance, they might run an anti-smoking ad because no one disagrees that smoking is bad for your health, etc.

CBS's rationale for this policy, said the spokesman, is their desire not to let groups with "deep pockets" control the public debate through paid advertising.

I can think of a lot of reasons why this is neither a good nor a coherent policy. But that's their explanation of it.