I'm fairly well travelled and in my experience it only seems to be the Americans who hire the most obnoxious, rude, unsmiling and impolite people they can find for their immigration and customs staff.
You haven't been through one of the hellholes masquerading as airports in the UK recently then, have you ? Frankly, the UK is BY FAR the worst. Worldwide. Rude and primitive security staff ("shoes off, guv"), inefficient immigration, largely incompetent staff, lousy airports.
In contrast, my recent experiences in the US were largely positive. Even flew out of BOS on Sep 11 (
). Arrived at the airport with plenty of time, expecting thorough searches. Thorough they were, but carried out in a polite and professional matter - in under a minute.
Perhaps I'm lucky, but I - so far - never had any serious hassle entering or leaving the US, and I do this on a pretty regular basis.