"Ad blocking", when done by the web browser, is a bit of a misnomer. It's more like "Ad ignoring", really. All it is is when the HTML is parsed, the parser identifies potential tags that lead to ads (using the height and width of standard banners or the server it resides on) and simply refrains from making the request.
Anyone suing someone else for ad blocking is going to have a tough time if the blocking is done entirely on the client side with the knowledge of the user. No matter how you slice it, ad companies can't prevent me from running whatever program I want on my computer. Besides, ad blocking and cookie blocking can't be illegal, since Junkbuster has enabled ad blocking for years and Konqueror has had cookie blocking for close to a year. Mozilla has both. None have been served legal notices.