From https://asahilinux.org/about/

> Apple allows booting unsigned/custom kernels on Apple Silicon Macs without a jailbreak! This isn’t a hack or an omission, but an actual feature that Apple built into these devices. That means that, unlike iOS devices, Apple does not intend to lock down what OS you can use on Macs (though they probably won’t help with the development).

Then why did they go out of their way and made the bootloader able to load another OS?

To get a better idea where their security holes are.

So the opposite of a Trojan Horse? A Greek Gateway?

More like a Honeypot.