If you're referring to a carrier's zero interest payments on the full price of the phone, that's just smart. No penalty for borrowing. If there was interest involved, it gets fuzzier.
I'll amend this by saying that there's also another huge requirement for it to make sense: The new iphone must be necessary for any of this deal to make sense. If it was impulse bought because of this financing offer, then the financing has cost you >$1k
Last I checked, my carrier (T-Mobile) charges like 10% more than Apple for the same device. They're just baking the interest into the price, it's not really "no interest". Maybe the others don't do that, though, I dunno.
I'll amend this by saying that there's also another huge requirement for it to make sense: The new iphone must be necessary for any of this deal to make sense. If it was impulse bought because of this financing offer, then the financing has cost you >$1k