I'm going to try to keep this short. I get annoyed with the meme that Americans have lost their work ethic and that other countries have developed or retained some kind of amazing cultural affinity for work. The difference observed in these cases has basically nothing to do with culture, to the extent that culture has does influence this America has an amazingly pro-work culture and has higher than expected labor force participation, hours worked, and entrepreneurship, and a lot to do with economic opportunity.
The problem is the belief that people should work hard, just because well, people should work hard. The corollary of this is often that people should damn well do what they're told for what I want to pay them or go to work for themselves (which ain't easy, even if you're sitting on your rear sending article after article to publishers). I could present this view more sympathetically, but I find it elitist and classist with no real supporting evidence so won't bother (the most recent article to make me think about this was in the Economist, which discusses immigration and the difficulty of finding Americans willing to do the work immigrants did).
The real reason people work is that they aspire to something better. People generally have a pretty good idea of what their opportunities are and will generally act on them. If people aren't taking jobs like vegetable picking in Alabama it's probably because they have a good idea that they won't get anywhere by doing it. Even poor people realize that spending a summer picking veggies in a scorching field won't do much to bolster their resume to get a better job later one, they're not stupid.* There's no upward potential, so they won't do the work.
Now, of course, many of my educated fellows can share stories about how they did unpleasant or backbreaking work to get through college or in between jobs. Hell, I can share a few of my own.
But there's usually a difference here, almost all of us with these stories were young, most often single, and had some kind of in kind supports that allowed us to save from these jobs. In these circumstances, most people can and do work. They do this because they know it's a stepping stone to somewhere better. I used to spend 8 hours a day getting yelled at in a call center to save for graduate school, while perhaps not as bad as working in an Alabama field it's pretty depressing and I got through it by telling myself I'd never have to do that kind of work again.
But for your average down on their luck unemployed person, this work doesn't look so good. That time in the fields means not applying to jobs that might get you out of your situation, people don't do this kind of work to simply tread water. At one point in time there may have been a path up the ladder, do good, get recognized, become a foreman. There's probably still some cases like this, but these days you're more likely to be passed over in favor of someone with a 2 year business or 4 year agriculture degree. You don't suffer through a crummy job with no prospects for another year at a crummy job with no prospects.
Looking at countries like Mexico or China, the situation is different. The Chinese maintain, at considerable expense both in terms of state dollars and economic distortion, a range of state owned companies that give vastly inflated wages and benefits. People swarm into their cities and take crummy jobs in hopes of eventually being able to qualify for one of these posts. Without them, it's unlikely that people would work for the wages and conditions they are given in Chinese factories, but people have a rational expectation of being able to move up and out. This won't come true for many, but it comes true for enough to keep the dream alive. Mexicans immigrating here have similar ideas, they may eventually learn English and move up, or at least their children will.
This simply isn't the case in the US right now, most low wage jobs are insecure, often seasonal. You don't move up the ranks if turnover is less than 3 years or so, which is the reality for most low wage employment. Unions once made up for this by negotiating scheduled wages, this had its downside but people will put up with a lot if they know things are getting better every year. But we've gutted unions, we demand degrees for most positions of responsibility, and wages are too low to save for school if you either have dependents or can't rely on family or a buddy to let you sleep on their couch and save for school. In this situation, why bother with a crummy job? It gets you nowhere, doesn't build a resume for a more responsible position, and barely covers a subsistence level of income.
Of course people don't do these jobs. Society is never, and will never be, set up in a way that people will work hard without prospects. When people did back-breaking farm work it was because they knew the economy was growing and more responsible, better paying positions were opening up. People will work for that hope, even if it doesn't come true for everyone doing it. But that isn't our modern economy, it's very rare that someone can take a labor or retail job and move up. Who would spend their lives stuck in a low wage, insecure job? No one would, surveys taken show that both the poor and middle class have remarkably identical opinions on what they consider the minimum wage they'd work, the same could likely be said of opportunities. This is simply a reality that we'll have to deal with, people won't stand for the scraps thrown to them by the rich and the only solution is to create better expectations, not to lecture people to become resigned to having none.
* The exception to this is the recent graduate who has never held a full-time job. Most managers have dealt with enough bozos that skipped shifts and called in "sick" (hungover) to be somewhat leery of taking a chance on a middle of the road student with 0 work experience. In this case, an entry level crummy job is pretty much a prerequisite for something better later on. It's always been thus, and recent graduates have always moaned about it. This, however, doesn't seem to be the case with much of the unemployed, particularly in the low wage sector.