Many people, especially older ones and people with bad or old monitors don't even realize they are ads. There's purposefully no border and a light background is used to confuse people into clicking ads.
I'm pretty sure this isn't intentional. No one at Google is going to be using a crap monitor. I've used similar colors in business software before. It wasn't until I was at a conference and saw our software on a crappy screen that I realized the required field color was white on some monitors.