public service announcements vs. advocacy advertising.

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does anyone know what the difference is between "public service announcements" and "advocacy advertising? i was intrigued to find that most US networkds have explicit guidelines which they follow regarding what ads are acceptable or not. most ads dealing with political or social issues are deemed to be inappropriate "advocacy" and not shown. yet, at the same time, i thought that the networks were mandated to use a percentage of their airtime for showing public service announcements. what's the difference? why can you say "stop smoking, its bad for you" but not " stop buying huge cars, its bad for society and the environment"? and why can you get an emmy for developing PSA, but there are no clear guidelines so you know it will be shown to any/all networks.