While the "Reddit's /r/{something} is supported by people with a financial interest in {something}" can be distasteful, under what theory of law would or should it be made illegal?
If you are saying that someone with some stock in AAPL and would be pleased if it went up can't be a mod of /r/apple and should have criminal liability for moderating a community's activity (and would likely ban people who consistently post 'Apple is bad, buy Dell instead' or 'Avoid apple - Steve Jobs was a bad man') then you're suggesting a different thing that would be harder (in my eyes) to argue for.