Macs allow the device owner to install an OS that isn't signed at all, without having it degrade the security of the system when you do boot into MacOS.
Fine, but can it be disabled? If secure boot is interfering with another function of the computer, the owner might decide they prefer hibernation over secure boot.
I think what you're missing is that "secure boot" isn't a system-wide on/off thing on a Mac, it's a per-OS thing. And UEFI Secure Boot specifically is something that only exists on a Mac to the extent that Asahi shoehorns it into a system that doesn't natively do anything UEFI-related. It would be very surprising if Asahi Linux didn't still provide a way to skip their UEFI Secure Boot code paths and just plain boot.
Yes, but that's not a perfect excuse since OpenCore (and Clover) exists. macOS very well can boot without iBoot's opaque "man behind the curtain" blobs, Apple simply never entertained it as an option on their chips. Apparently important stuff is happening in that boot process and they can't have you emulating it for fun or profit.
That is worth discussing though, as it's a marked departure from old Macbooks that did support the UEFI method.
