Is it kosher? Pretty much -- lots of app developers already pay for advertising that's targeted towards app installation, and this deal isn't any different. Rather than pocketing all the profit, Apperang is just passing it on. A video on the site shows that Apperang is promising US$.25 per app install (if the app isn't free, they'll reimburse you the purchase price), and their FAQ says that the minimum payout is $1. Payout appears to work through Paypal -- that's the only option I can see so far. For some companies, it's just easier to pay a quarter for a new user rather than gamble on advertising or word-of-mouth.
I do question how scalable it is from Apperang's perspective, and of course, as a user, there are much better ways to make some pocket change