I'm not sure if this is a pricing error but it only applies if you select the iPhone 6S 16GB model. It works with any iPhone/Apple Watch colour variant (+$3 for 42mm Apple Watch).
(Side note: there is no price difference selecting the 64GB model v 128GB model, perhaps another pricing error)
- Lowest plan offers 1GB data, unlimited talk & text, and unlimited music streaming for $95 per month | $40 plan + $55 devices ($37 phone and $18 watch)
- Second-tier plan offers 3.5GB data, unlimited talk & text, 150min international talk, and unlimited music streaming for $83 per month | $65 plan + $18 devices ($0 phone and $18 watch)
All other iPhone models have a logical pricing scheme, e.g. $102 for 1GB, $109 for 3.5GB, $118 for 7GB. It seems the $83 plan wipes out the month handset fee.
The iPhone 6S Plus 16GB has a cheaper third-tier plan too (plus Premier League subscription included), but the device pricing is still logical (that is, decreasing in price).
ADDED: As undefined pointed out, you can get just the iPhone 6S 16GB on the $65 plan with $0 handset fee.
16GB in 2016 should be a crime.