To be provably fair it has to be provable and any of its members can verify it. There is no probably fair casinos those are probably scams every online gambling website that is successful is provably fair.
the question is where you put the emphasis on the term honest. If the games are fair, but a casino does not pay out, then you are still dealing with an unfair casino, while the technology and game technically are put together correctly. It is a combination of both for the total understanding of honesty.
You're right BTCGOLD if the casino doesn't pay your winnings or doesn't let you withdraw your funds then its provably fair feature won't protect you as a players. Moreover if the game is provably fair but the user doesn't change its seed, the house can use non random server seeds and patterns.