Subsidised hardware. You get it cheaper because it constantly throws you at Amazon. Fine if you use Prime and watch their stuff/read their books. Not great if you want a standard Android tablet with access to all of the apps. Ideal for older people I guess. Or Gary Barlow, Jimmy Carr et al who...
Still limited to the Amazon store rather than the Google store - thus missing out on many applications, I guess?
Edit - just checked. They say 150000 when there are millions on the Play store so - yes.