They modify their getwork server to report less rejects/stales.  There's no mystery there. I can do the same here if it'll make you feel better, though.  I can, in fact, using their methods, give you 0 rejects.

They have the same reject rate as here, they just don't report it to you, so you will never know if you are having a problem or not and therefore causing a detriment to the pool as a whole.  On an individual level, it's not a big deal, but when you start getting a lot of miners, and if everyone has absolutely no idea if there's a problem, it can add up to a significant drain on the pools true hashing power (vs reported hashing power).  Unless they've done some mathematical gymnastics with stat reporting, their true hashrate is 1.5 - 3.5% lower than their reported hash rate.  That means you are losing out on 1.5 - 3.5% of your potential income vs variance when you make a decision to mine at a given pool.  (That is not to say your overall income is materially affected, only your decision making ability to assess whether one pool is better than another.  Although, if enough people have unreported problems it will also affect your overall income as well.  But you'll never know unless people self report a lower than expected income.)

They bill this as a "feature," and in so far as they pay you on stale shares it is definitely a bonus, but in reality it's basically dishonest math when measured against the accepted standard.  I've not found anywhere that they confirm or deny that they report stats based on stale shares as well.