I see the issue after digging in more. The problem is that Robinhood claims they make money by investing in fund with extra savings, however they failed to address how they are "really" making the money. I always doubted how they make money and now I know.
What's even more surprising is that other company like E-Trade costs money AND they sell it to HFTs.
What's even more surprising is that other company like E-Trade costs money AND they sell it to HFTs.