Their code is closed source, and with assumption that casino might be cheating, it would require some big amount of money to test it, because even if you lose once or twice it doesn't mean anything, if it happens 100 times in a row, than it becomes suspicious. So obviously no one would waste their time and money trying to catch some dice site on cheating. That's why those sites feature provably fair system - cryptographic proof that each bet is fair and wasn't tampered by any side. The bigger risk with dice sites is that some sites might promise huge jackpots to lure people into chasing them, and refuse to payout if someone wins that jackpot.