I am guessing microchip are thinking something like lets not try to be the cheapest solution but lets be the easiest whole cost solution. As a pre-verified module the RN2843 is probably going to save you a lot of money on the product RF verification stage, maybe it's 5-10K savings. So they probably figure that's a lot of sensors you need to catch up if you use a cheaper non-verified module where you have to implement and verify the stack. So that's something like a 1000 sensors your going to need before you compete with their more expensive but 'gold standard' sensor.
By making the serial GPIO on the device you can just bind a very low cost microcontroller to add some application logic ( even a 10F family ) for just a few cents and use the GPIO on the RN2843 to read and write IO for "Sleepy" sensors. ie the low bandwidth sensors they consider to be their sweet spot.
In the future I wouldn't be surprised to see them also allow programming of the onboard PIC18F46K22 - or even a flash upgraded version. I for one would very much like to see that.