All bitcoin gambling sites have this same problem. All dice sites. All card game sites. All casinos. The solution is just another problem (and will reduce speed, and reduce profitability.)
Not all. If a casino is bankrolled by a single person, then this isn't an issue.
As soon as the owner gets help bankrolling the site, however, he is able to steal from his investor.
I had the idea for Just-Dice about a year before I started work on it, and put it off thinking "who would invest when they know I could cheat them". I tried and tried to find a way to make investing provably fair, and eventually just gave up and did it anyway.
To my surprise nobody seemed to care that I could in theory cheat them. And even now, when the profits are only 0.61% of the wagered amount when they should be 1%, people still trust me. So that's nice. Basically the site has earned a lot less than it 'should' have, and I could easily have stolen the difference. I didn't, and people trust that I didn't. Mostly.
Since 'celeste' took around 4k from the site, the profit has pretty closely followed expectation (the green line below is 3.9k below expectation; once you're 3.9k below expectation, the expectation is that you stay there...):
A couple of weeks ago I heard a rumour that some people unknown were planning to "out" me as being 'celeste', the whale who won on Just-Dice. As far as I know, nothing ever came of that, so I don't know what that was all about. Maybe they're saving it until their own dice site launches or some such. So there's something to look forward to.
