Why do I have to activate a new phone with a carrier?

Episode 1220 (1:04:30)

Randy from Seattle, WA
Apple iPhone 6s

Randy's wireless provider is an MVNO, or a third party carrier, and the phones he gets are unlocked as a result. But he wants to buy a new phone and keep his existing account. Leo says he can, as long as it's GSM. Leo suggests going with T-Mobile because it's unlocked out of the box. Then he can use it with any carrier.

The iPhone is different because they require activating the phone with a carrier first. Leo says that's very weird since Apple sells mostly to the main four carriers and it would be a burden to have to activate a new phone and then cancel your account. It's likely that Randy is just getting the wrong advice from the carrier or salesman. Leo did it and he had no trouble just putting his SIM in and using it.