What is "cloaking"?
According to Wikipedia, cloaking is a search engine optimization (SEO) technique in which the content presented to the search engine spider is different from that presented to the user's browser. This is done by delivering content based on the IP addresses or the User-Agent HTTP header of the user requesting the page. When a user is identified as a search engine spider, a server-side script delivers a different version of the web page, one that contains content not present on the visible page, or that is present but not searchable.
Why cloaking the ads?
Google recently hates 'deceptive ads'. Please refer to this article to find out more about what deceptive ads are.
Sometimes, you don't have a choice. You have to place advertisements to keep your website or service running. But, Google may flag your website for having the deceptive ads, then they will drop your website's ranking.
To be clear, cloaking the ads is different from cloaking content. Cloaking the ads does not modify the main content on the page. Theoretically, it's a safe cloaking technique.*
* Yes, theoretically. Because this is an experimental project. We can't make sure that it will always be safe and working as expected. Google algorithm evolves.