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.
The non-retina MBP is still available. Or at least it was up to a few minutes ago; they've just taken the store page down for updating, I wouldn't be surprised if the old MBP is gone when it returns. Although you'll probably still be able to buy one until the stock runs out.
There's still one left (I just got one). The pricing model of the Retina MBPs is just craziness to me. To get larger storage sizes you have to jump entire model lines to get it, you can't just upgrade storage on them.
Just for the older Retina models. Copied from the link you provided: "Specifically, the "Mid-2012" and "Early 2013" models use a 6 Gb/s SATA-based SSD whereas the "Late 2013" and "Mid-2014" models use a PCIe 2.0-based SSD. These SSD modules are neither interchangeable nor backwards compatible."
The reason for the sentence you quote is to explain that PCIe chips are different than the proprietary-pinout m-SATA port in 2012-2013 Retina MacBooks. They are strikingly similar and if you're not paying attention, you might try to move one into an incompatible machine.
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:
I was thinking long and hard about maybe getting a refurbished 15.6in MBP. I ended up with a HP Omen gaming laptop instead, because I could get it with a 512GB SSD and 16GB of RAM for $1700, instead of about $2500. I'm giving up battery life of course, and then there's the issue that it runs Windows...
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.
