Plugins on the activation (sometimes for the 1st time only) usually add custom capabilities exactly to the ‘administrator’ role to provide for admin a full access to a plugin.
If user can install new plugin, he can get a superadmin privileges in a minute adding to the site a plugin with a special PHP code. There is no sense to set any restrictions for such user, as he can overcome them in any moment.
New plugins should be installed by the person fully responsible for the site – superadmin. Even if user will not try to become a superadmin, adding new plugin may break the site – so it is a potential large problem for the future maintenance.
“as such user is a superadmin there.” – superadmin is a “God” for this site. He can anything. There are no restrictions for him inside existing functionality. There is no sense to limit him in any manner.
