So what's the deal with cell phone companies.

Why is it that you can get a much better deal on a plan, with more data if you sign up and buy your phone and plan online, rather than in store?

Going to switch to Verizon prepaid plan $45 for 8gb high speed data, and for a limited time the'll throw in another 8gb. That's what I read on their site anyway.
Go into the local Verizon store, nope, only get 8gb.

And, they sell the Motorola G6, and the cheaper, less impressive G6-play.
You can only get the G6-play if you go with the prepaid plan. Why wouldn't they want to sell the more expensive phone regardless if your going prepaid, or contract?