Why would they bother? They don't need to cheat - there's no way to beat blackjack or any online casino. You simply lose your x% every bet on average.
What confuses people and makes them imagine both winning possibilities, and "cheating", is the enormous variance. Feed some numbers in here:
http://www.winnergambling.com/casino...ce-calculator/
eg Cryptologic standard, $100k total wagers with $100 individual bets, on average you lose $442. However, within two standard deviations are losing $7.2k, and profiting $6.3k. There's a 45% chance you profit, 55% you don't.
Remember, these sites want addicts. They don't want to drive people off. More than anything, they don't want to get a reputation for being fixed (and with a large sample size logged, it's easy to prove a site is cheating through basic statistics). If words of that gets out, they're screwed in terms of attracting new mugs. Much more profitable in the long term to play it fair.