The entirety of China doesn't even have Google Play Store.
"If the Google Play Store app still isn't showing up, contact your carrier or manufacturer for help. "
>Why doesn't Google allow it to be installed on all devices?
You can install it on devices (with varying difficulty depending on if you can unlock bootloader). It just doesn't come pre-installed on any device sold within China.
When media reports security bugs for Android, it is hardly clear whether the bugs affect AOSP or the Google branded Android, and to what extend.