Under the definition of duress within contract law, this is the only section I feel applies here:
Lack of reasonable alternative (but to accept the other party's terms). If there is an available legal remedy, an available market substitute (in the form of funds, goods, or services), or any other sources of funds this element is not met.
http://en.wikipedia.org/wiki/Duress#The_elements_of_economic_duress
However, it depends what you consider reasonable. To me, driving is a reasonable alternative for shorter trips, but anything over 500 miles and it's not reasonable. But then I'd also weigh in the cost of the trip. If it costs the same or less to drive than to fly, regardless of distance I'd consider it fairly reasonable. So it's a matter of preference for this one and what a court would see as reasonable.
So given it's a case of do it or don't fly, for international flights I can completely understand the 'duress' aspect. But for internal flights I'm not so sure, especially for a country like Britain where driving anywhere isn't that bad.
I'd also add here that it's no different to a number of other situations, such as the nightclub example people keep mentioning. With the nightclubs it is a case of be searched or bugger off. Now given every club I can go to involves this pat down pre-entry procedure, there is no reasonable alternative and yet no one complains about it.
As I've said in other threads, it seems like Americans have double standards. You are happy to accept searches for clubs and not make noise about them but the moment it's something regarding the government it's the worst thing ever and must be stopped.
From my perspective, being searched before boarding a
private aircraft is no different to being searched before entering a
private club. The only difference between the two is who does the searching. Perhaps you would prefer if the airport had it's own security, privately funded by ticket price increases, that you agree to on the ticket between you and the airline, who searched you instead?