Why is it that by far the biggest majority of Yanks I've met have been good humoured, good fun,?

Unfortunately, US Border Patrol and Immigration aren't supposed to be nice. A lot of people want into the US, and many do not have good intentions. Their job is to stop them. They're even pricks to Americans (with the exception of San Ysidro, CA where they are extremely hospitable compared to other places).

And at most you deal with them for a few minutes and then you are on your way.

We are pretty fun people, you're right :)

The immigration people's job is to be mean. I'd probably be mean, too, if I did that crap all day. Think about it. Their work day consists of hundreds of people trying to lie their way into the country. They have to deal with people trying to smuggle stuff in, make up relationships to get green cards, you name it. It's a crappy job. Same with all the authorities. They deal with useless people all day long. I'd be pissed too.

Maybe because you Limey's called the immigration officials Yanks.