Better yet, why isn't all cartelization prohibited?
Unless we abolish corporations as a concept, how exactly do you effectively propose that we "outlaw all cartelization" in a way that isn't just outlawing unions while keeping capital interests unified?
tl;dr: unions complement the inherrent concentration of capital interests in modern ecconomies and are no more inherently like cartels than the corporate form itself.
You don't have to write some sort of universal program that can take a textual description of a situation and spit out "guilty/not guilty". You can make laws stating a thing, which a bunch of motivating text, and then direct prosecution against things that are against the spirit of the thing, and get judges to follow it (either through rulings, or if that fails, adding extra laws to make it even clearer what is up).
It's a social thing, not a logical contradiction gotcha. "Corporations may not engage in anti-competitive behaviour. Corporations who act as such shall be liable under anti-trust statues as if they operated as one. Also, price-fixing is not allowed." Then society and the rest fills in the blanks