The API of a BIOS is not the whole thing! It just specifies the interface by which the BIOS can be called by the OS.
For a crude analogy, consider the interface specification that most modern cars adhere to: there will be a steering wheel with a certain shape, size, and position relative to the driver's seat; there will be two or three pedals, with the accelerator on the right, the brake pedal to its left, and the clutch pedal, if present, to the left of the brake; etc. etc. None of this tells you whether the car runs on gasoline, diesel, CNG, or electricity, or whether the motor is in the back or the front, how many cylinders it has, etc. — all these are implementation details that do not usually affect the interface much.
The implementation of the BIOS — the code that actually performs its various functions — is clearly copyrightable, and always has been. But the API is just the specification of what those functions are and how they can be invoked. Imagine if one car manufacturer had been able to copyright the steering wheel and pedal arrangement that we're all familiar with!