Are you under the belief that the very entities (market makers) that have the best visibility into the instantaneous supply and demand of various stocks, the ones whose entire existence involves running the markets, matching buyers and sellers and ensuring fair price discovery, the people who are best equipped to profit from speculative trading, don't also play the markets for extra profits under the table? That's what they want you to believe because it would be a conflict of interest if they had open positions that were not 100% hedged in balanced manner.
They would like you to believe they are just the referees who don't have an interest in the game, but nothing could be further from the truth. Who do you think keeps the profits that happen "inadvertently" through the course of their market making activities? They are not immune to greed, nor do they have high moral standards, they manipulate the crap out of it. That's why they keep the most relevant market data to themselves and only release information to create the appearance of open, fair markets. If they didn't want the trading edge, then please explain why the short-interest data is so opaque, so tardy, so fragmented, and so incomplete. It serves no justifiable purpose. And that's just the tip of the iceberg. The less hedging they need to do, the more money they make.
Welcome to Wall Street, the world's biggest casino! This is why I am primarily a buy and hold investor, it reduces the advantage they can take of me to the absolute minimum and greatly increases my profits over the long-term.