It depends on how the carrier includes those applications on the phone. If they include the apps in the actual ROM image, a la Gmail/Browser/etc, then they cannot be uninstalled because they're part of the core ROM. If they include the apps installed in the user's data partition, just like any user-installed application would be, then it's uninstallable by the user.

Neither methods preclude the carrier from using the other method, and in many cases, carriers will use both. eg, Sprint has their unremovable NASCAR app as part of the ROM, yet also includes an uninstallable demo of Need for Speed on the user partition.