Decrypting Devices...why not?

I've read from a number of individuals on message boards stating that once you encrypt your device it's not possible to decrypt it fully without a factory reset. I'm a bit confused by this, especially considering that the option to decrypt the device is clearly present under security settings (GS2 phone) and, sure enough, the device decrypts just fine. Can someone explain this for me, please?

EDIT: Perhaps this is the source of the misconception, or perhaps this is a more device-specific thing? See this page which explains my experience accurately: