Because then: whose computer is it? If it's my computer, it does my bidding. If a child is currently on the computer, should the computer decide and refuse to work as in "I'm sorry Dave, I'm afraid I can't do that" where the operator (Dave) is deemed a threat to its objective? In this case, the computer's programmed goal would be preventing any minor from seeing content with this tag. It has to decide if you're a minor whenever content is marked as such. If you can't override its programming, it's your computer no longer

You could configure the user account to show tagged content or not.