But at the very junior / basic level of officer it seems to be a very unskilled job with very little decision making or discretion. So basically, scan passport, take fingerprints and ask a couple of meaningless standard questions and then either admit the pax or refer him/her to a higher authority. Why does every single one have to be a US citizen over here with all the expenses an ex-pat would require?
Now the more senior immigration officers that would conduct interviews (sorry, interrogations) with anyone flagged up by the junior front-line ones may well need to be ex-pat US citizens. But wouldn't you need only one of these for say every 10 front-line ones that actually sit in the booths.
Solutions may well exist but it takes a will to effect any change...
They are "law enforcement officers" similar to a warranted police constable. Many will be ex army or ex police. I can't see it working so easy in the UK - imagine how long the MCTs would have to be and what the conformance time would have to be. A lot of front of plane travellers would prefer to risk a lengthy wait in the US than mix with the great unwashed in a CBP post clearance zone (no duty free, no lounge etc).