They are not entirely wrong, in that the way the machines are set up in China make DCC the default solution and the manoeuvring to get it to charge in RMB is not always straightforward depending on the equipment. Some merchants or hotel reception staff may well in good faith believe that this is how the bank has set it up even though there is in fact a way to cancel DCC. IIRC, There is detailed discussion of that issue on the China forum and what can be done to avoid it.