TINSTAFL. Shop owner pays for the POS terminal, pays for the phone connection, pays some percentage of the transaction and even for the printed logo on the bill. I don't believe any business owner would pay for that without charging clients for the cost, so in the end, it's split out onto the products prices. Specifically in Poland there's a law that f.ex. cigarettes cannot be charged more than the MSRP, but the rest of products obviously can, so in the end, you are the one who pays for the comfort of paying with plastic.
Also, cash is billed directly, money from bank transactions come with a (afair 2-3 days) delay, so obviously paper gives the business owner a better cash flow than plastic. Oh, and a very-cheapo Polish 7-11-alike Biedronka after many years of operation afaik still doesn't accept cards, and besides slave labour I think it's also a matter of being able to drop prices a bit more. So there's probably some common sense there. I don't like being unable to pay with card though.