What's your citation on that? I don't know of any such law. Aside from political ads, a licensee has very broad control of what is and isn't allowed on its airwaves, including ad content.
This goes back decades and decades. I recall a Koteen & Burt client letter in the 70's stating that "if you take any ads in a category, you must take all".
The origins of this came from companies like Bonneville limiting the ad categories they took. When Arch Madsen (a personal friend) was head of that group, they got "opinions" on their decision to not take beer and wine as well as cigarettes in the 1960's.
The premise was that "if you discriminate against one brand you are committing an unfair trade practice". A major station that refused, let's say, Pepsi ads could be sued for hurting Pepsi's business by allowing Coke and 7-Up and Canada Dry but not that one brand.
In Puerto Rico, the broadcaster's association petitioned the NAB to make an exception to the NAB Code to allow rum advertising as rum was a core part of the local economy. NAB said "no" to rum, but that we could carry all hard liquor ads, not just rum. And later, the NAB Code itself was ruled to be a form of collusion.
So the situation where stations could not take one account in a category but not another is based on trade regulation, not FCC rules.
The only exception would be if you questioned the creative, such as "we don't take screaming ads on our Beautiful Music station". If you apply that uniformly, that is a legal practice. That's your case of "content".
But if you take the Chevy dealer and not the Ford dealer, you could be sued. Trade practice, not FCC rule.
