When you go to Network manager applet 1.20.0 to view the WIFI network and go to your WIFI SSID you will see a tab called wi-fi security, then there is a check box to see the WIFI password. If this is clicked the WIFI password is displayed without a admin password being entered. You should always have to enter a password to see the WIFI password, this exists for Mac OS and windows.
You would be suprised what else just gives out secrets in cleartext without authentication.
This is the prime example why I steer clear of GUIs. Not that this can't happen on CLI, but UI development is often times a couple of sizes too large for hobby developers.
I'm pretty sure I remember that 'nmtui' does not have this issue, which is an ncurses cli interface for NetworkManager.
Dont use Network Manager and use something else to connect to your wifi, or better yet lock your machine when away from it so no one can view it.
The issue is not with NetworkManager. It's the frontend to it. Besides that we don't know to what administrative lengths the OP went in regards of file permissions.
Nonetheless I believe that a lot more people should use NetworkManager, and this GUI in particular. Blow issues out of proportion, don't let these devs rest on subpar work.