I think it is a national security issue mostly!
The trouble to register the phones is far less intrusive IMO than the problems that exist if they are left unregistered which consists of enabling terror, drug deals and other criminality. We are not in a pre-9/11 world anymore and we can't IMO have unregistered phones in these times.
You know ... after the terrorists' plot in London, the terrorists getting captured in the USA (6 of 11 so far) and the cellphone outage in NYC this afternoon .... I'm agreeing with you.