If that's the case, wouldn't that imply that a huge class of transactions for basic life necessities -- rent, most bills, etc -- be under duress as well?
That seems like a useless and counterproductive interpretation to me.
"Under duress" seems to be a somewhat fuzzy classification, but most of these necessities have either very low costs for switching to another counterparty (if I don't like one grocer's offer, there are plenty of others around I can go to instead on the spot) or some limitation on what they can require (e.g., no kicking a tenant out without a month's notice). At-will employment seems to be the odd one out here, in that switching to a new job normally takes quite a bit of time and hassle.