Why can't I disable an item?

Some StartupItems do not contain the information used by the system to allow them to be toggled on and off. The systems uses a configuration file, "/private/etc/hostconfig", which contains lines like "MYSQL=-YES-" which tell the SystemStarter that the service "MYSQL" should be launched. Many StartupItems use a script to launch a service, those scripts will check to see if the service is enabled (the SystemStarter makes the information from the configuration file available to the script), if so the script will launch the appropriate service (in this example MySQL would be launched). StartupItems do not have to be based on a shell script, however. As long as there is an executable file (be it a binary application or a program written in a scripting language such as AppleScript or perl) with the same name as the StartupItem directory the SystemStarter will launch it. It is the programs reponsibility to determine if it's service is enabled or disabled.

To be able to properly enable/disable a StartupItem the item must use a shell script to launch a program.

System services are controlled by the SystemStarter in MacOS X. The StartupItem does not have to support the ability to be disabled via the system configuration file, thus not all StartupItems can be toggled off. For more information see Apple's SystemStartup documentation.