Because their (in)dependence was questioned on various fronts in the context of the american bipartisan system and more specifically the republican party and it's policies aka "the party that hates them".

Wrt the subsidies, consumer market and all that i don't have much to add but wrt the migrant workers the point of contention to my knowledge is mostly illegals (regardless of the actual number deported, the perhaps brutal way in which this is done, etc), ICE, etc. It's also my understanding that illegals are far far more present in farm work and a few other industries in the US (and to lesser extent in europe) to the extreme extent that those without legal work authorisation make up nearly half or more of the farmhands. (USDA estomated 42% few years ago but others had good reason to suspect between 50 & 60% or a even more) So yeah there's no real way to not think of illegal farm work there.

In that context and the opposition there's some elements like Bernie that seem to stick to their line and call this kind of faux open borders a right wing position whilst the rest of the democrats and their base seem to kneejerk the other way in response to recent events and republican standpoints and suddenly seem to have started supporting illegal entry, employment, etc