New to Kindle, relatively new to Android, but as to why they did it, pretty simple, the carousel (especially the recommended apps/books/media you see unless the feature turned off) all lead to Amazon's infrastructure. If you buy something through Amazon, the profit is mostly theirs (minus author/developer cut obviously).
If you were using Google Play/other app store/isohunt for APKs, Amazon would see no money. Given the profit margin (or possible loss, depending on model and price it was sold at!) on these devices, they are banking on making money from you with future purchases.
That it is how it seems to me at least, sure others will chirp in should I have mis-read the situation.
(Happily side loading apps, Calibre for my e-book collection, but not yet rooted my Kindle Fire (2nd Gen).)
Must update my sig sometime.
Kindle Fire 2 (CM10.1.3 Stable)
Samsung Galaxy Ace 2 (CM10.1 nightly)
NoName Chinese WinCE6 GPS (ceFFM replacement interface)
HTC Touch Pro 2 (UK unbranded. Energy ROM 6.5.3 . Dodgy digitizer & keys)
HTC TyTN II
/ Kaiser (UK unbranded. WM 6.5 custom ROM. Screen & some keys knackered)
HTC Hurricane,HTC Typhoon,iPAQ 2210,Sony Ericsson K700,SE T610,SE T68,Sony T65,Sony CD5e,other bricks