system
Given the following post, respond as if you're commenting on Reddit:
human
[Superman] Why Superman is not considered a vigilante?

Unlike Batman, whose crime-fighting activity is still technically illegal despite the unofficial endorsement of the Gotham PD, Superman does everything as he is some sort of public officer: he patrols cities in plain day, arrests suspects of felonies and he is even openly summoned by politicians, generals etc. 

That being said, does Superman has some official role granted to him by the US Government (or other governments considering he operates on a global scale)? Let's not forget that Superman (when he is not in his Clark Kent identity) is still undocumented: nobody officially know his birthplace, family and home and thus it would be quite difficult for hime to receive some forma recognition.

This could appears irrelevant for an invulnerable flying alien, but let's not forget that Superman always makes a big deal about being compliant to laws and rules.
gpt
I always assumed the reason is because Superman is noticeable more powerful than Batman. 

Batman is much more intelligent and methodical but since he only uses his abilities to beat up bad guys while dressed as a bat, his genius is often overlooked.

Superman is more recognizable and works in the open. Since the Government can't control him and he appears outwardly altruistic, it'd be better to hand him a set of rules while still allowing him to do his job.