Its more of a mentality thing.
Take Ultimate Reef for example. There are hundreds of tanks on there, mostly from the UK and a few from Euro and its mainly UK guys on there and no one batters an eyelid when you see a Regal Tang in a 4ft tank. Why? again comes down to mentality, I have a 4x2x2 bow fronted with a small regal in it. For the time being he is fine and no problems, I have 3 tangs in the tank at the moment, a yellow and a kole keep him company.
If i posted my stock list on here people would have a fit, literally. But I do not have any problems, readings are all fine, there is very little fighting other then some chasing away from hidy wholes which I doubt you can avoid. All my fish feed and are healthy, nothing more then I can ask for.
Same goes for alot of tanks I see in the UK where having a powder blue in a 4ft is considered normal and again no one would batter an eye lid. Lipsticks I see in 3-5ft tanks regularly as well.
My father in law has a Regal in a 5ft tank and has had it for 3+ years so far and its a very healthy fat fish, nothing wrong with it.
Somethings are just plain short sighted, like a sailfin in a 2 ft tank, but even then I would consider one in a 5ft tank if it was over 2ft tall. But then again across the pond the accepted school of thought with those "in the know" is that you are doing a misjustice if you keep one in under a 240g tank.
The impression I and others I know have is that it is just more acceptable this side of the pond to be more daring with stocking levels, that goes for both size and quantity of fish.