He's making the implicit assumption that two corporate forms (you know unions are corporations, right?) are equivalent. A for-profit corporation has a collective mandate to make a profit for shareholders and a union has a mandate to make a profit for union members.

More precisely, he's asking what moral/political philosophy principle he can use to distinguish between corporations selling labor and corporations buying labor. Zedshaw suggested "the law says X" as the underlying principle which is a bit unsatisfying.

(It's unsatisfying because it implies that, e.g., Rosa Parks and similar individuals were in the wrong when they broke the law.)