What's the difference on month 25 if you don't upgrade? That's always been annoying to me about contract prices: your phone bill didn't go down when your contract was up and presumably your phone was paid off.
I figured most customers wouldn't notice to take advantage of this which is why I think a clearer separation between phone payments and cell service payments is a good thing.
IIRC, they were pressured into this by T-Mobile's offering, and Verizon now offers something similar for the same reasons.