They don't turn a blind eye. It's just that the VPN's that work through China's firewall do so over ports that the entire internet uses, like 443 (https). They could filter everything, but it'd be a loooot more resource intensive to filter all internet traffic by destination/content instead of only doing that with extra ports that extra services like VPNs normally use.

... and as others have pointed out, there ARE valid reasons to allow some VPNs. Forcing them to jump through hoops to function is no skin off their back, while properly filtering all traffic would be disasterous for them beyond the expense.