They don't.
World science fiction conventions are autonomous and independent of one another, apart from (in theory) having to abide by the WSFS constitution. There is no permanent floating worldcon organization that oversees these events. It has worked up until now -- through 81 conventions in a variety of countries -- because everyone abided by an unspoken agreement to observe the rules.
The Chengdu concom appeared to be following the rules right up until they didn't any more and went "rules? what rules?"
Here's your brief intro to what worldcon is:
https://en.wikipedia.org/wiki/Worldcon
(Disclaimer: I'm a three-times Hugo award winning author and the author with the most appearances on the Hugo shortlist from outside North America.)