The issue is security. If you lost your phone, you probably wouldn't want someone to gain access to all your stuff simply by turning the camera app on. If bypassing your security code / pattern / whatever were that easy, then there wouldn't really be a point to having it in the first place.
When you use the camera, what actually happens is that you are using the camera app, with all its features. It would be difficult to completely separate the camera app from the rest of the system and let you use just that, especially when using the camera also gives you quick access to your gallery, files, etc. It would open up a serious vulnerability, so if you want your phone protected then you have to unlock your phone before you do ANYTHING. I for one prefer it this way.