Obviously don't read the T'snC's about their card. As the banks randomly decide on when a CnP payment is required. Some banks is totally random and some banks are when you done £x or certain number of transactions, whatever comes first.
It's the bank, NOT the shop/cafe/wherever that decides this. Please don't yell at the staff who are powerless, have no ability to lift the restriction up. Also if customers found they have the cash or pay with another card, the first card will need a chip n pin payment before you can use contactless.
People need to understand that they don't have to keep the random generated PIN that their bank issued to them. People can go to any ATM and change their PIN to something they will remember under PIN services. Some banks refuse customers to change their PINs to 1234, 1111 etc