An embassy is no different, for example the American embassy in London, they are Americans, often in uniform, performing the role of their government on another countries soil. Why should it be any different for certain other agencies such as immigration.
These setups make life easier and faster for those that wish to use the service.
I did receive a notification through work regarding pre-clearance at Shannon and my understanding was that it was for biz jets too. I would suggest advanced booking is probably required however.
The aircraft I fly is only just trans-atlantic capable, what would happen if you have to put down in Canada on your way for a quick top-off? Would you then have to clear immigration again?