For the gambler provably fair only means you have a tool to verify your bets. If they were skipping nonces, changing server seeds, etc you would still be cheated. Only if you check all your bets, you can find out that you were (or not) cheated.
For the investor there is no way at all to verify anything. In theory it is very easy for the owner to fake a winning whale with the server seeds (or just small winners .. every day.) I actually started a discussion about that
https://bitcointalk.org/index.php?topic=774458 "Provably fair for investors?", but the only solutions so far are not really provably fair, just slightly less risk.
Thanks, this is an interesting, and I think, more pressing problem.
Attempts to cheat the gamblers are always detectable, whereas attempts to cheat the investors are not. If whoever inserted the dicebitco.in skipping code had simply stolen the server seed and cheated more subtly, we'd be no wiser today and simply wondering about the string of bad luck.
It would be interesting to look at the Bitcoin gambling sites that allow investment and look at the return on investment : is it always less than the expected return?