You're presuming merchants made a conscious choice. In China I don't think imposing DCC requires consent of the merchant - the banks can impose DCC unilaterally.

Who will stop them? Visa? The CBRC?

I did do a search. I haven't been able to find a Chinese advertisement of DCC. Maybe my search skills are bad or maybe this is pushed by relationship managers by word of mouth, but I have not found any publicly available proof of that.

But it's also just as likely that the banks pushed DCC without explaining to or obtaining the consent of the merchant. Replace the terminals, tell nothing to the merchant and make it impossible for the merchant to turn DCC off.

Visa has a pretty crappy relationship relationship with the local banks who will do nothing to enforce Visa rules (think of the spit fight they're having with Visa-Unionpay cards abroad). Either Visa "does a Google" and turns off Visa acceptance for all of China, or they let their visitors get ripped off. With so few cardholders actually aware of the problem it's not difficult to see that Visa can't be bothered to get tough on the Chinese banks either.