Isn't this a useful form of price discrimination, though? The coupons allow you to sell your products at full price to those who are price insensitive, while also allowing you to sell at a discount to price-sensitive buyers. Only the latter are willing to go to the hassle of looking up the coupon codes.
My wife installed a plug-in on our family computer that looks up coupon codes for you at checkout time. But often they don't work, so I don't even bother with them. To me, that suggests that even if its 'easy' to find the coupon codes, the coupons still work as a form of price discrimination.