I think the really simple fact of violation in DCC is the Bank of Communication Moondog mentioned. It's system chooses DCC by default, and your slip says you are offered a choice, which is so not true.
I have no problem being offered DCC, but I want it like Hong Kong where you can see the rate and you hit the button, so you have the control.
The Ritz Carlton in Beijing told me the bank charges them 1% of non-DCC, and 0.5% with DCC. I am not sure what the rate is for UnionPay transactions.