One reason for this is that in Chinese society this sort of thing is fairly commonplace. If you head over to the local bazaar shopping area, you can find the latest brand name clothes being sold for a fraction of the price you pay back in the US. Of course, that Prada bag you get for $15 is fake, but the people selling it to you don't care, and they assume you don't either. Quality may vary of course, but the look is the same at least, and that's what matters for some people.
Pirated DVDs and music are available too, copied and sold for you for about 1 dollar each. You might miss out on the behind the scenes documentary, special packaging, and good graces of the record companies of course, but surprisingly few people anywhere really care about that.
What people really care about is law. In the States, law has a relatively sanctified feeling. People stop at traffic lights even when no one is coming the other way. People often walk all the way over to the intersection to cross the road instead of jaywalking. Obeying the law, in general (I know there are big exceptions to this) makes people feel safe with each other, and many Americans see any law, even a small one, as a black and white issue -- you either follow the law or you break it.
Some laws in China, however, are gray. It all depends on which law in particular you are talking about -- certainly some are taken very very seriously (like not killing or robbing people; Chinese law enforcement seems to take these very seriously, and city-dwellers tend to feel very safe on their streets, even at night), but others (like... copyright) are a bit less clear as to exactly what the law even is, to the causal observer.
So to really care about something like "Terms of Service," which are just rules of the company, not even laws of the government, just isn't high on the average citizen's priority list. Going to all this trouble to ferret out gold sellers and such would just lead to people finding ways around it anyway, they might say, so why go to all the trouble?
Once the expansion comes to China, it will be interesting to see if Blizzard's operator in China, The9, applies the same in-game tools to fight the gold trade and spamming that we have seen in the US with the arrival of patch 2.1. Will Chinese start to actively oppose gold sellers once they are able to report and silence them with a mere click?