No. That's not us only, but there are definitely unsupported carriers.

However, it's not "Free". Amazon pays the developers every minute you use their App, in exchange for you keeping Amazon's full App installed (Amazon's Android app is pretty much an ad platofm), encouraging you to buy more from Amazon and rely less on Google Play. If you remove the Amazon App, every underground app will refuse to start. Every time you update one of these apps, you have to open Amazon and inevitably see the front-page. Also, although rarely, you might be shown a full - screen ad before starting the game.

Also, consider all the permissions the Underground app has. Also consider all the Amazon ads you see online. Having their app installed and actively collecting massive amounts of data to trace back to an unique advertising ID is both Amazon's and ad-networks' wet dream. Developers sign up to publicize their App and they get paid by time. Amazon has to pay pennies to maintain those Apps and they can accept / refuse your app freely. Bonus points: Amazon Underground by design needs to collect analytics about your app usage habits in order to pay the developers - and nobody tells you they stick to tracking what you do with their Apps only. Take Quality Time, a single permission gives it access to how much time you spend on all your apps, globally, and very in-depht. Once again: advertiser's wet dream.

TL;DR: It's not free as in developers and Amazon give it out; but it's a situation where everyone wins: the developers, Amazon, ad platforms and end users