I did learn a new word and concept in the reading. That of "underinclusivity". The explanation will cause you to get mental cramps. Something about if the restriction tends to do less towards accomplishing the states' interest than if they did nothing at all??
What underinclusivity refers to is a restriction that includes a bunch of exemptions based on the content of the speech being regulated. If the exemptions defeat the purpose of the restriction, the restriction fails to directly advance the governmental interest. The best example was the federal ban on advertising casino gambling but exempting Indian casinos. If the governmental interest was minimizing the social costs of gambling, that interest was defeated by exempting Indian casinos.
The plaintiffs argued that the city's restriction was underinclusive because it banned only off-site advertising (billboards) will exempting onsite advertising (signs). But the city's restriction was there to further public safety and aesthetics, not to regulate the content of billboards or to ban advertising. The city wanted most of the billboards gone. The ordinance directly accomplished that, so it wasn't underinclusive.
I'm still trying to figure out why the exemptions to the Do Not Call registry don't defeat the purpose of the registry.