Cloaking refers to the practice of presenting different content or URLs to human users and search engines. Cloaking is considered a violation of Google’s Webmaster Guidelines because it provides our users with different results than they expected.
Some examples of cloaking include:
– Serving a page of HTML text to search engines, while showing a page of images or Flash to users
– Inserting text or keywords into a page only when the User-agent requesting the page is a search engine, not a human visitor
Now, in this specific case, the webmaster only wants to show ads to users who are logged in. Users not logged in, do not see ads. Since GoogleBot cannot log in, GoogleBot will see what a non-member (not logged in) sees, which means no ads.
In this case, since both GoogleBot and the non-member sees the same thing, then it should not be cloaking.