There's a lot of Potentially Unwanted Programs or PUPs that todays better antivirus will warn against. This install toolbars and more. Here's a link about PUP.
If you look at the rating for coupon.com you see they are mostly OK but they do the dang dirty thing and block you using the back button on the browser which is a sign they are not all that great a site. -1 in my book.
In my email using Yahoo I always receive coupons from coupon.com. The last two times when I open email from coupon.com and enable link I receive a message from AVAST that the file gu.aspx is a new or unusual file and that I should proceed with caution if I download it. Other info given is it is from frontdoor1coupons.com and there is no digital signature. Is this a virus or should I download the link to get coupons? I have AVAST FREE and have not downloaded the file at this time.