Prime customers are Amazon's highest spending customers. They, according to this report[1], spend more than 2x what non-Prime customers spend - $1340 / year. Prime customers aren't likely to be swayed by a low-cost, budget phone. So it makes sense for them to tie their premium Fire Phone to this segment of their customer base.
This also doesn't mean they can't do a budget phone later.
[1] http://bgr.com/2013/12/17/amazon-prime-spending-study-cirp/