Yepp, the reason is that you are not registered with Amazon. At the moment, the only way to create collections on your device when NOT registered is, to use the Kindle Collection Creator.
I do that because i consider a registered device a severe privacy threat. Beware though. As soon as you register your kindle (again), it will delete all collections created by 3rd-party software with the ones stored with amazon (yepp, your collections are stored there).