I've heard they generate most of their revenue through an online sportsbook. www.wsex.com Below is their answer to your quesion.

I am in no way affiliated with this site. However, I would like to see it do well since that would pressure other online poker sites to lower their rake.

How long will World Poker Exchange be "No Rake"?

We are doing everything possible to make No Rake a permanent change, one that will firmly establish World Sports Exchange and World Poker Exchange as the "go to" sites for the online gambling community. We made the change with the intention of keeping it no rake. If the market happens to make dramatic changes, we may have to revisit our business model, but for the foreseeable future, World Poker Exchange is a no-rake site. We can do this because we're confident No Rake Poker will deliver new traffic to our other businesses, including sports wagering, live betting, horse racing, and casino wagering.