Farmers may be another special case, actually, now that it has been mentioned. Unions do exist, and are good for the smaller farmer, to give them some defense against massive corporations and a better position negotiating with stores and whoever else buys their produce (not talking about actual individuals going and buying a sack of potatoes or whatever) but on the other hand - to what extent is that viable in the long term? Might we be better off helping small farmers get fair prices for their land and ensuring they get support to retrain or the like? Ultimately society wants absolutely vast amounts of mass produced food at the lowest possible cost, which is something the massive corporations or similar bodies are genuinely very good at. Even if part of that is on the strength of illegal immigrants.

There are other issues that make farming a bit special too. First is food security. No country much likes the idea of putting its food supply into foreign hands, although the modern world makes that pretty much inevitable, there's still a sense that being able to feed your own people (at least for a limited time) is a strategic consideration. Second, farming culture is very strong in many parts of the world, even as fewer and fewer people are actually farmers. Our species is built on the strength of twelve millennia of farming. It's only in the last two centuries that we began to change to a less farming-based society, and only in the post-war era that it really has changed. In many countries it's still the primary vocation. So we have a strong attachment in the historical and cultural sense, and many modern cultures idolize farming and the rural life to some extent; look at the American homesteader, Tolkein's idyllic Hobbits, or the French belief that the farm is a vital aspect of French culture which must be defended and preserved (which, in turn, forms a big reason for their consistent defense of the CAP). Many places are still very tied up with farming as an important economic activity, so whilst here in Leicester, UK, our farms are far away now, in Bozeman, MT you're probably not further than ten minutes from a major farm or ranch of some sort. If Leicester lost the economic benefits of its farms I don't know that it'd be a huge issue; if a small Great Plains or Mountain town in the US did, it'd probably be game over.

I'm not arguing that and of these factors should necessarily should inform decision making, but I can see why it's a sticky subject and hard to discern what the consequences of various policies might be.