Tt's exploitation when you don't have a free choice. Sure, we can choose our exploiters, but what kind of choice is that?
Do you hate the capitalist who is the only person who is willing to employ you and in exchange, he will provide you with warm bed and food?
There's two choice, really: work or die.
At that point, the notion of exploitation is irrelevant. The capitalist is saving my ass. In return, I have to work in his sweatshop.