[number_man]

> When I travel to China with a mobile phone subscription that supports roaming, I get completely uncensored access to the internet.

This is because when you are roaming, the data traffic goes through an encrypted tunnel to your carrier. There's not much China can do without completely blocking roaming traffic. It's not a big deal though because vast majority of Chinese people don't have access to a foreign mobile line. As long as China can control what 99% of the population can access, it's already a win.