Microsoft sort of has to do this. The margins on these devices are going to be thin as it is, and an OEM won't want to produce them unless they can control the scheduled obsolescence of the device and force the consumer to purchase a new one in some future upgrade cycle. If you could just download Windows 9 or 10 yourself, attach your tablet to your PC with as USB cable, and update the OS on the device, then the OEM loses that control.
With x86 tablets, they're likely going to have at least one full USB port and a BIOS that allows booting from USB drives. So if you can get Windows setup onto a bootable USB thumb drive, nobody can stop you from updating the OS yourself. For this reason alone, x86 tablets are more than likely going carry a higher price tag than ARM tablets.