The device becomes yours once the rep finalizes the transaction, and not before. When you shop at any store, nothing you are carrying around belongs to you until the item is paid for and the transaction is finalized.
Once the Genius hands you the device, and you get a receipt e-mailed to you, the device becomes yours. Until that point, the device remains the property of Apple.
He is suggesting ripping a phone out of an employees hands, and not allowing them to do their job. You tell me, if you're at work doing your job, and someone stops you halfway through forcibly, and tries to finish it for you, will you continue to help that person?
It is the stores responsibility to take care of the customer, including activating a phone replacement. If you and the rep are both okay with you completing the activation, that's fine. But forcibly removing a phone that doesn't belong to you from someone's hand while they are trying to complete their job is far from okay.
You don't have to agree with how they do their job, and that's why they have managers you can speak with. If you want to activate the phone yourself, just ask. If they refuse, ask to speak with a manager. If they also refuse, then you're SOL at that store.