I feel like I'm missing something here... Is it not legal to run an investment management company as a non-profit? Likewise, is it not legal for the non-profit

I feel like I'm missing something here... Is it not legal to run an investment management company as a non-profit? Likewise, is it not legal for the non-profit arm of Vanguard to mange its own investment as an integrated part of its organization? At what point do these legal distinctions begin to take effect? If a friend and I pool resources and purchase some stock, then later sell it at a profit and split the trading fees, did we somehow owe unpaid tax? It seems like this is a tax against vertical integration in financial services, which I'm not sure I understand the need for...