For one thing it's bad because of information asymmetry between the market owner and other participants. The market owner knows all the information but each other participant only knows a fraction of the information. If the information was all completely public it would be less bad probably. This is why in financial markets "dark pools" (private alternative execution venues) ended up getting new regulation from the SEC to prevent abuse https://www.cfainstitute.org/en/advocacy/issues/dark-pools#s...

Antitrust law doesn't come into effect just because something is bad in the abstract sense though, it comes into effect when there is harm to the consumer as a result (that's my understanding anyway). The harm here is due to the market distortion primarily (I would think).