I don't see how it's conspiratorial at all. There are market forces that are influencing these decisions, and there will always be exceptions. It is a generalization, but it's a stochastic generalization based on real life data - not on the assumption that there is a secret cabal of evil suits smoking cigars in a dark room.
I'm not a Marxist, but I don't see how Marxism is a conspiracy theory. There are certainly some conspiracy theories on the left sure, but Marx never assumed that there was a secret conspiracy trying to control things behind the scenes.
Regarding small businesses preferring cheap labor, it depends. Take the transportation industry - if you're a small business owner who works in the transportation industry you're likely to be an owner-operator - in which case you don't really care about cheap labor. Likewise in the hospitality / retail industry you're likely to hire American workers if you're an American and non-American workers if you're non-American. So the cheap labor from immigration only impacts you indirectly. Obviously if you're an immigrant you're going to be more likely to lean to the left, but must small business owners are American.
It's true that the construction industry benefits from cheap labor, but we're talking about general trends here. Someone who owns a Yoga studio is more likely to be left wing. But on the whole, small business owners lean to the right.