I mean, these websites don't have to listen to Google if they don't want to. Google isn't a law enforcement agency. They just have to follow guidelines if they want to use Google's advertising service. This is far from the first guideline Google has for its advertisers.
It's not just good publicity. It's good customer service to its users. They already have guidelines about how advertisers can't have copy in their ads that are misleading/advertising something other than what they're actually selling/etc. They don't want their users to get caught in "gotcha" moments, because then they'll associate that bad experience with Google. And then Google will end up with unhappy users, which eventually means fewer users, which eventually means fewer advertising dollars.