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.

http://forums.cnet.com/7723-21578_102-553218/what-is-pup-in-avast-free/

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.
Bob