I wouldn't call Reminders a part of the OS, since it's just an App. I'd call Siri a part of the OS.
My sister has an iPod Touch and doesn't need Reminders, she doesn't want it, and she asked me how to install it. I had to tell her to just put it in a folder.
When I say "part of the OS" I mean that it not put there through a carrier requirement. Meaning when they build the OS, these "apps" are included by default in the build.
Just like the Android market, or the Gallery is on an Android phone. If you build Android from source, those two apps are there. Create a emulator through the SDK and those two apps are there. They are "part of the OS" in that they are "default apps" at the source of the OS.
Now, if you'd like to agree that the Gallery and the Android market are also bloatware then ok, we can reinvent the definition.