Why aren't phones generating their own keys when they're activated at the store? Burn a fusible link if necessary. This would be more secure _and_ cheaper for the carriers. Oh, because NSA has plants on the GSM committees?
No, because the subscriber identity is linked to the SIM card (it's in the name...),
and not to the phone. You can switch a SIM card into any phone (some simlock
issues excluded) and just keep going with your one subscriber identity.
Or put another SIM card in your phone and use a completely different one.
It's great when traveling.
It's a feature - it's even a "we actually want this" kind of feature.