You have heard of USB storage right? If you can afford a Surface Pro, a fat USB key is probably something you already own several of. I have 3 on the desk in front of me including one large capacity high-performance USB3 device for transferring image files for work and such.
You heard USB storage is slow as molasses compared to an internal SSD right. If I'm buying a Pro then I want it to run x86 apps, likely productivity apps many of which consume a lot of space .. USB expansion storage is a band-aid and ultimately not a solution.
Yes you can do x, y and z tweaking and removing recovery partitions and uninstalling some bloat but most people outside a tech forum don't know how to do that and are stuck with whats given to them out of box - thus the backlash about lack of free space.