US MM's may not be traders by SEC law. That would put the fox in charge of the henhouse.
Remember, every trade is logged as to buyer, seller, mm, brokerage. If an MM buys a stock for which there is no buyer, he is breaking the law, and the data is present to prove it. It strains my brain to think about penny stock mm's actully going hands on buying and selling for themselves. MM's are huge companies, and the amount of 'profit' they could make by trading is zilch compared to being busted by the SEC, and perhaps losing millions of dollars in revenue.
Besides, it is an automated marketplace. MM's don't even have eyes on these stocks - except to change their spreads. The MM's competition is other MM's. If they misjudge their spread, a different mm with a better spread will fulfill the order.