Poker sites in particular trade off having a good reputation. If one were discovered doing anything unethical, they'd lose most, if not all of their players.
Gas stations price gouging aren't going to lose them many long term customers, if any.
It's a pretty ridiculous assertion that a site would look random in the long term but "vary" the randomness in the short term. When you deal 3 or 4m hands a day, you'd be changing this setting for about 2 minutes before it becomes statistically obvious should anyone be tracking a significant portion of those. Two minutes isn't going to be affecting anyone's bottom line.
Also, two independent audits of the PokerStars shuffle algorithm:
http://www.pokerstars.com/rng_audit.html