The issue is that usually the manufacturer builds in capabilities to update the firmware, and then reserves for themselves the ability to modify and update that firmware, while intentionally denying that ability to the users. This puts the manufacturer in a position of power over people who use the product.
But if the firmware isn't modifiable by anybody (including the manufacturer) because the capability to update it was never built in the first place, then no capabilities are being withheld from the users of that product.
