BIOS authors typically end up with documentation from hardware vendors and a Leading Alternative OS vendor which describe their expectations of the BIOS, and so BIOSes are often written based on those assumptions. The problem is that Linux vendors (even large ones) rarely get to see any of this documentation, even if they have NDAs with the hardware vendors. We're usually left trying to determine the nature of the expected behaviour by looking at shadows cast on walls. So the answer to your question may be "Because the hardware and OS vendor told them to", or it may be "Because their crack was cut with meth that morning". We just don't know.