It's like they somehow expect millions of letters to the editor begging them to stay.
You do know we are talking about a British man who lives in Britain? He's not Alec Baldwin, forever defiantly leaving America and wiping the dust off his feet. He is British and lives in Henley, and has done (as far as I know) for decades. Why should his British passport be invalided - in the eyes of American passport authorities - by the fact that he was born in America?
If Catherine Zeta Jones - born in Britain - were to visit her homeland, would she be held at customs like Boris? No. So why is it ok the other way around?