There are two plausible explanations for this:

1. Incompetence. The same reason why many banks al around the world do this without regulations. Some snake oil salesman sold them a security theater SDK or library that blocks user installed or modified OSes.

2. Government control and surveillance. Vietnam is authoritarian. It only makes sense for them to participate in the global war against general purpose computing to gain complete control over their citizens' devices allowing them to restrict software, displayed content and communication to require government approval and enable total surveillance of all activity without any way to bypass this. Instead of outlawing user controlled general purpose computing directly they do it through the backdoor of pretending that it is for people's own safety.