Why do some manufacteur's list BIOS's to download and flash on their website then.

Take a look at this,
https://www.asus.com/ca-en/Graphics-Cards/ROG-STRIX-RX570-O4G-GAMING/HelpDesk_Download/

Scroll and find

Version - 2017/05/09
1.42 MBytes

Quiet BIOS for ROG-STRIX-RX570



Since its listed on their website you obviously are allowed to flash the bios, and most people save a backup so they flash back the original bios instead anything goes wrong.

So I don't think its right for them to void warranties due to a simple bios flash.