Well, it's not just a legal question. The problem with colluding is that it involves effectively taking money from the non-colluding parties. And this causes those parties to not want to play anymore. If a small or medium investor know the algorithms are going to cheat them, that investor would not want to play the game and this, illegal or legal, prevents investing.
You have to keep in mind that markets are both a "game" and way that capital can be mobilized by society for productive purposes and one doesn't want to kill this latter aspect.
You have to keep in mind that markets are both a "game" and way that capital can be mobilized by society for productive purposes and one doesn't want to kill this latter aspect.