The article doesn't say you have to sign up for a plan...
... just that you have to activate the iPhone Air through either Verizon or AT&T. For those who don't have Verizon or AT&T (i.e. they have T-Mobile or an MVNO), they only get $100 off instead of $200.
I understand this to be similar to how Apple charges $30 more if the buyer wants to activate and connect to a carrier later.
View attachment 2618183
This is what Best Buy says about them activating the device for you to get the $200 discount.
View attachment 2618185