They're making less per game than they would charging a lousy buck on the appstore. Given all the ways they can make whole dollars, as in plural, per purchase I'd say it's not better than 0% at all.
This is free advertisement for indie developers, who have a very hard time getting publicity. Not only does it pay off in the short term (many of these are users who I can promise you would never have bought these games for anything more), it also pays off for the companies in the future. Wolfire, the creator of Lugaru, is currently working on the sequel, Overgrowth, which they have been funding exclusively by using preorders of the game. If someone buys Lugaru here and enjoys it, they may just preorder Overgrowth, which will net them more money.
It's the same with Frictional Games (creators of Penumbra) and the game that they're funding through preorders, Amnesia.
It seems like pretty expensive advertising actually - a normal sale makes them $x minus the ad or whatever that brought them the customer. Now $x is somewhere between $0 and $1.58 where it used to be up to $20 minus whatever advertising.
They're not making more they're just selling more.
World of Goo did the same thing a while back. They made an incredible amount of money and reached a large amount of people. Yes, many people paid 1 cent, but they still made more than they would have otherwise.
Not disputing they're going to make a lot of money from this, obviously when they wake up tomorrow morning they're each going to have tens of thousands more than they had when they woke up this morning. In that sense it "works".
But relative to other models the "pay what you want" is a terrible failure - any of those games could have singlehandedly grossed $30,000 to $60,000 from the number of sales they've had today, and that's going to lose a big chunk to advertising but still make them a lot more than $1.66 a sale.
