SSH login as root gives you root access to the Linux operating system running the MyCloud NAS.
root has a lot of power ("with great power comes great responsibility"...). If, logged in as root, a user enters a stupid command (such as cd /; rm -Rf *), WD are really not responsible, and the resulting dead NAS is not their fault, and therefore not a warranty issue; it's the user's fault. It's like buying a new car, and then deliberately crashing it into a brick wall and then trying to claim that the car should be replaced under warranty...
One could argue that WD should not allow SSH login, but I'm happy with the compromise of allowing us root access, but saying if we do something stupid as root, it's our fault, and our mess to sort out.