That's not an uncommon practice. I believe there was a non-Retina Macbook Pro in the product line for quite a while after the introduction of the Retina Macbook Pro.
https://www.google.com/shopping/product/12203990123520892001
http://www.everymac.com/systems/apple/macbook_pro/macbook-pr...
You can still swap a drive in newer models, just with a PCIe card instead of an mSATA drive.
Simply put: Newer generation of hardware = newer storage interface.
Here is a replacement guide for a 2014 model Retina MacBook Pro:
https://www.ifixit.com/Guide/MacBook+Pro+13-Inch+Retina+Disp...
However, I've been pleased so far. The keyboard takes a little getting used to (because it is in the exact center of the laptop), but is good for typing overall. Good feel, no flex. The display is good too, with a wide viewing angle.