On iOS, if you have the 'Limit adult content' restriction enabled, then it becomes impossible to enable or disable Safari extensions while this restriction is in effect. This allows developers to create effective content blockers to restrict inappropriate content.
I expect the same functionality on the Mac platform, such that if a standard user account has parental controls and 'Limit adult content' enabled, then it should be impossible for that user to disable Safari extensions. However this is not the case - the resticted user can enable / disable extensions freely.
It this a bug or intended functionality?