It's all too easy to slip out of the sterile areas in many airports. I did it by accident and pure ignorance once (before I was a FF) - lots of red faces all around, but it was whispered that it wasn't the first time.

In places like HKG, NRT, ICN, etc with a significant proportion of transit it may make sense to build your airport from the ground up to support a secure, sterile transit.

In the USA, transit was only ever such a tiny proportion that it could never make sense to do so. It would be a good idea to put in place a fast track for transit pax at the few airports where there are many; if it was enough biz for the airlines to care then doubtless they'd pay for it.

LHR T5 has fingerprinting of domestic pax in order to be able to support "secure" transit (and I wonder just how secure that really is). I regard that as a darn sight less "civilized" than forcing transit pax through immigration/customs.

All that said, with such a porous northern border, the security paranoia seems rather beside the point. But if you are going to have immigration/customers in an airport, it makes sense to do the job properly.