The technical name for this is fraud. It occurs frequently in the area of business and trade. A merchant can rob a customer by falsely labeling his goods with regard to weight, count, and quality. A customer can rob a merchant by deliberately incurring an unpayable debt and declaring bankruptcy. Other common forms of fraud include entering false information on one's tax form or falsifying an insurance claim.
Most coupons clearly state that such a practice is fraud.
In the eyes of many people this kind of robbery is a very mild sin, if indeed it is considered sin at all. Thus even Christians are often guilty of it without fully realizing the seriousness of their deeds. In principle, however, it is no different from any other theft. Even though many people are doing it, and seem to be getting away with it, it is still wrong. The eighth Commandment forbids it.