In reply to rztipping:
> It's more common there as they mainly drink keg beer. Keeps better and full of co2 so won't go flat quickly.
> We like our cask beer with live yeast. Doesn't last long once dispensed.
*beer geek klaxon*
AIUI the main thing with keeping is whether it's in an oxygen-free environment. If you just fill up a lemonade bottle with any beer straight from the tap then the beer will start to oxidize and go off a bit after a few hours (although I guess you might notice less with a fizzy, strongly flavoured keg beer), whereas if you flush the container with CO2 before filling and capping it then you're probably good for a few days. Most places that do takeaways as a major thing do the latter, and the machines to do it are mostly set up to connect to keg probably because very few people outside the UK cares about cask ale very much so the market for a cask version would be pretty limited.
Anecdotally, another reason I've heard that it's big in the US is that beer-oriented bars (and particularly brewpubs) are often quite hard to get to without driving, so if you want to drink something that's draught-only then getting it to take away can be the easiest option. In the UK it's easier to get leathered in the pub and then walk or get the bus home.