Taking someone’s work product and converting it, without compensation and consent, into your profit is theft of labor.
That's not what's going on here. The finished product contains only the style of the artist that the AI was trained on, and style is not copyrightable. Which is a damn good thing, as humans have been learning from each other's "work products" and mimicking each others' styles since time immemorial.
BTW, theft of labor means failing to pay wages or provide employee benefits owed to an employee by contract or law. You're using that term incorrectly too, Greg Rutkowski wasn't hired to do anything for the people who trained the AI off of his work.