The preinstalled apps are part of the android system. Or part of the phone-provider you are using. it that case they are custom-roms. Even the original from google is a custom one... The decide some apps not being deinstalled... there are restrictions about this apps or something.
I dont know a way to set an own app on this "security-list" or whereever this may defined.
You (may) can - its not as difficult as it sounds:
1. set the depending system directories to R/W instead of R/O
2. copy and install SU (SuperUser) to the system
3. execute SU and set the rights for it to root
4. allow SU to manage the root accessment
5. copy and install YOUR app to the system
6. remount the depending directories from R/W back to R/O