You do NOT need to register if you will be flying indoors only.
This is not an ownership registration but rather an intent registration. You "Intend" to fly outdoors...therefore you are commanded to register.
While I realize that few will fly indoors only, it does mean that if you are wanting to wait to see the outcome of any potential litigation concerning this registration, Or, your Phantoms are in storage or inoperable or you simply want more time to decide if you want to submit to the registration, you can wait....and just not fly "outdoors"