As much as I completely understand why you’d want this, I think it would be a lot of hassle for the developers and, thus, waaay down the list of priorities.
Amazon’s use of the open source version of Android means that developers can’t easily use Google Play Services. A great deal of developers rely on that automatically updated application because of the sheer number of devices that can use an app that’s designed with Google’s APIs. I believe that a lof of apps on Amazon’s storefront don’t keep parity with their Google Play counterparts because of this.
It’s relatively easy to install the Google Play store and everything on an Amazon tablet. I did it last year on mine and it works fine. I’ve had no problems with things not working and I can get any app now on my Fire.
But in general, I think the FT app requires a phone because it links your phone number to the account.