Not sure I'll ever understand the reasons why shopping malls don't allow photographs, but I really can't imagine it has to do with security.

Since you're on private property, legally they can ask you to stop taking pictures. What they can't do is touch your camera. You have no obligation to tell them why you are taking pictures, to show them the pictures or to even tell them your name. They may of course escort you off the premises.

It is completely legal to photograph the mall from outside when you are standing on public property.