No Junk Mail - Unsubscribing from Snail Mail

At the bottom of every promotional email from a legitimate company, there's almost always a link that allows the recipient to unsubscribe and thereby not receive any future newsletters, emails etc from the company. Why can't these companies have something similar for their snail mail? They can enclose a little postage paid self addressed card that the recipient can send back to the company should they want to opt out of receiving future promotional mail. Alternatively, the companies can offer an option on their websites, where members of the public can go to opt out of receiving junk snail mail.