Yes the reason is that Amazon does not think you can be responsible with your own purchases. Essentially Amazon will not sell you a car with horsepower that will take you over the speed limit. I expect the GA-510 to be pulled from Amazon shelves or not replenished with new stock.
I live in the United States where my freedoms are protected, I can legally buy whatever I want when it comes to electronics. If someone is a not a responsible person and use their electronics recklessly, that's a discussion for another thread. I've proven for over a decade that I'm a safe driver and I use my electronics in a knowledgeable and respectful manner.
I think the 13r is the old version, 13 pro is the new one. 13 pro can only be programmed using the new software from these type of hardware devices, called CPS (customer program software), chirp isn't compatible yet but hopefully will be soon. The CPS app has the ability to modify the firmware of the device, which Chirp cannot do. This is how other variants of the device exist (MU-5, GM-30), simply the firmware has been modified to tweak the hardware to comply with the regulations targeted, like a speed or lane limiter. The UV-13 pro has no locks or special firmware, you are permitted to do anything you want because you purchased the silicon. You can program the device using the keypad but it is kind of difficult, using the PC software is a breeze. Once you have your personal save file it takes less than 30 seconds to set up a radio.

 

Thanks for the info and writeup Aaron!