As for what they're expecting? This is a genuine Scottish tabloid newspaper cover from a couple of years back:
http://current.com/entertainment/wtf/88862229_i-kicked-burni...
There have been attacks on British airports in recent memory: not often, but often enough to justify the permanent armed police presence. And then there are the security nightmares everyone in the security biz remembers, such as the Lod Airport massacre:
Hell, I've seen policemen with machine guns in the subway. (I'm not a gun person, by the way, so maybe "machine gun" isn't the right term, but it's definitely the kind of gun that doesn't require you to pull the trigger per bullet fired.)
TSA everywhere, but nobody carrying anything remotely rifle-like.
Probably terrorist attacks like these:
http://en.wikipedia.org/wiki/Rome_and_Vienna_airport_attacks
Last week, a friend told me that when he flew into Amsterdam some years ago, there were people with automatic weapons standing around, and he was told it was routine.
Can I ask what the hell Europe is expecting that the US isn't? Because aside from some deployments in the immediate wake of 9/11 (which I vaguely recall were such a joke that the rifles weren't even loaded), we don't do that here.