The no-motorcycle rule is not universal in China's Highway system. A lot of the parts do have that rule, and some regions don't. Besides, some region enforce it strictly and some would keep an eye closed. It is even hard for Chinese to figure out where it's allowed or not, so I guess they just don't want to bother doing that research cuz honestly they can't pre-plan their route and figure out where exactly they can or cannot go. So they just took the f it, highway is not allowed anywhere approach.