I can certainly see how investors might only want investors on the board! Screw the other owners! That would be sketchy!
You can find stations that break down like this:
CEO and CXX of company A is on company B's board.
CEO and CXX of company B is on company C's board.
CEO and CXX of company C is on company A's board.
Now, no direct conflict of interest, but you can see the issue. Especially when CEO of company A also sit's on company A's board.So, yea on it's own the co-founders on the board is not a big thing. But, looking into the board should be part of due diligence.
PS: You can also see issues when VC's make up the majority of the board, but that's less common with public companies. (Let's buy up little company X that my firm invested in.)
You can have a board focusing on balancing public good with profit, if that's what the companies charter says. But, that's probably not a great investment.