Is Spyware, which Is a Type of Advertising on the Internet Could be Illegal?

Spyware known as Advertising Supported software (Adware) is a way for the authors of an online downloadable product to make money without selling it to their users. Many large media companies offer them to place banner ads in their products in exchange for a portion of the revenue from their banner sales. So the user doesn't have to pay for the software and the developers are still getting paid.
In this case, U-Haul is suing WhenU because the company is complaining that they are losing thousands of visitors with all the pop-up ad's that are not from their own web site, people complain that they are annoying and frustrating that they have to click 'cancel' every time they visit a hyperlink on U-Haul's website. …