Google AdSense calls these "vignette ads". If you're an AdSense publisher, it's your choice whether to opt-in to this ad format, but Google strongly encourage

Google AdSense calls these "vignette ads". If you're an AdSense publisher, it's your choice whether to opt-in to this ad format, but Google strongly encourages it by constantly telling you how much money you're missing out on. If you do opt in, it's dead simple to add these page-level ad tags to your site -- simpler than trying to design the ad layout yourself -- so I imagine a lot of publishers opt-in reflexively. If you opt in, Google gives you a knob to control the ad load, and a way to run A/B experiments, but there's nothing reported in the results of these A/B experiments that lets you make sensible tradeoffs about the quality of your website. The system encourages you to turn the knob all the way up.