How can Firefox install WebApps without root privileges?

When visiting a supported site Firefox shows a pop-up message asking if you want to install a WebApp (for instance facebook, pandora, etc). If you accept, the corresponding package is installed (for instance unity-webapps-facebookmessenger, unity-webapps-pandora-com, etc) which among other things involves copying files under folder /usr/share/unity-webapps/userscripts/ (no root password is requested during the process).

My question is: how all this is possible since Firefox is running without root privileges? Couldn't this lead to security issues? Thanks

That WebApps are not actually applications, there are just links to the sites (like normal bookmarks) + special items in Launcher.

Also this "Apps" are installing only for current user, not for all users.

So no need to have root privileges to "install" them.