In many cases they do actually offer the discount (or tack on the "hidden" fee for using a credit card). In some markets there are also sellers who only accept cash (often discount stores etc.) and correspondingly have lower prices, which is a slightly more inconvenient version of the same thing if you patronize them.

Moreover, we should encourage every retailer to offer this, because getting 2% back (or less) while paying 3% more is not just a net loss, it's also worse for privacy.