It may be that it just isn't that strictly enforced for a variety of reasons. China allows a number of VPN services that bypass the firewall to function. My guess is that it isn't a huge deal because the vast majority of people don't care enough to go out of their way to bypass the firewall; the social effects of having that firewall are still in place.

Start enforcing it heavily and the people that DO use those services may start protesting or moving into activist roles.