Why is it illegal to choose who you sell to? That's not bullying their competitors out of their own market.
When you hold a monopoly the same rules don’t apply. That’s why it’s often advantageous to allow material competition in your own market because your adjacency influence can be outsized and cause unintended consequences -even if you normally would have been allowed to do such things-. Those unintended consequences don’t require malicious intent they just have to exist and be materially a result of your monopoly. The more your maliciously collude though the worse the remedy will be for you.