My understanding of how it works in radio is this: Say I manage a local radio conglomerate, and I have 4 stations all under one roof. 94.2 FM The Wolf - Hootin Hollerin Country 96.8 FM Pop Fresh - Top 40 Hits 99.6 FM Hot 99 - Blazin Hip Hop & RnB 1236 AM Sports Talk
It used to be legal for those placing ads to ask a radio owner to put a "no urban dictate" clause in an ad contract, meaning that if someone bought ads to run on all my stations, but had a "no urban" clause, I could not run their ad on Hot 99, as it targets an "urban" aka "black" crowd. Now, that is illegal, so a client can't say, well, we'll advertise on all your stations except the "black" station.
In TV, my guess is it would mean if someone wants to buy advertising for, say, a new snack chip, from Viacom, it could become illegal to ask them to place in the contract a "no urban" clause that might say the ad could not run on BET (for example). Or even Comcast, which probably manages quite a bit of the ad sales for TV One, might have customers that want to advertise on E! and Versus channels, but say, well, we have a "no urban" policy, so no thanks, we don't want our ads on TV One.
Another example might be an advertiser who goes to NBC and wants to buy ads on NBC, CNBC, MSNBC, Oxygen, etc., but says "no thanks" they have a "no Spanish" policy and therefore won't want any ads on Telemundo.
In my opinion, businesses are short sighted when they fail to realize that money is money, and you never know who might want to spend their money with you. Why cut your business off from potential customers?
As far as I understand, this does not really apply in cases where a business is doing more targeted advertising that should only be on one station or network. This is intended for bigger ad campaigns that are ordered to run across many stations or channels.