They're rarely right, though. A more realistic picture of life in extremely poor countries is: you don't have a choice between the sweatshop and subsistence farming because many of these countries are not agriculturally self-sufficient and there isn't enough land to support the current population as subsistence farmers. Instead, you get pushed into the city slums. These slums often have one very large sole employer, usually an MNC, which pushed everyone out of the market. You therefore have a choice between work and death; unsurprisingly people choose work.
This doesn't mean that this is somehow the optimal situation. For example, a more competitive labour market for labour employers would improve working conditions and lead to more employment - after all, the way monopsony markets work is by deliberately underemploying labour in order to drive the price of labour down and increase profit margins (think monopoly in reverse). So we have reason to want to be suspiscious of and even hostile towards large MNCs that undercut local markets and set up sweatshops for terrible working conditions. If you were to specifically buy from Third World small business and avoid MNCs, you can square the circle of: "how do we prevent horrific working conditions without depriving people of jobs?". Bad Samaritans is quite a good pop-economics read on this topic, for the interested.
