That makes very little sense. What does Apple get out of letting people run Linux and OpenBSD, then preventing it?

...to play devil's advocate a bit (because I really do agree with you), I could have asked the same question about Linux on the PS3, and we all know how that ended.

I run a “Hacker News advocate” against the comments I post, and this was one of the things I thought of. You will note that in that case Sony actually suffered quite a bit in their response, and they only really did it out of fears of piracy. In this case the I believe backlash will be fairly large, not the least from their own platform security people for having years of their work invalidated. Also, in this case the only thing I can see that could drive them to do this would be if FairPlay keys were somehow made available in the fully unsigned OS environment: I haven’t really looked into this (don’t have my M1 Mac yet) but I would be very surprised if so.

To have every open source app ported to their processor, and get the usage up. I'm a developer and I love the specs of the hardware, and I need open source tools to work on it.

On the long term though they can get more App store / cloud revenue by making sure that Apple's apps are always ahead in hardware compatibility.

Porting apps does not require them to open up their boot sequence.