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The ads come from an ad network where there is very little visibility into what's going to be displayed in your app. And bad people also keep managing to get their ads published even though the ad network doesn't allow them
And it all ties into the whole targeted advertising, where they also make sure very few people get the bad ad, and tries to target people they think may be more susceptible to these kinds of tactics. Depending on the amount of interactivity allowed, the ad can even display two different things if it deems you too savvy to fall for it.
It's basically unescapable unless you only use apps without ads, or pay for the ad-free versions.
The whole advertising industry is sketchy, more news at 10.