Simple. Domains are so cheap that if you buy one for ~ $8 then you only need to make that money back in one year for it to be profitable. That's less than 70 cents per month for a .com. There are a number of parking services where making that much money isn't terribly difficult. So... the more you buy, the more money you could possibly make. Some might just make their money back, some might do very well. Overall, the domaineers are likely at least breaking even. Add in the fact that they have a bit of experience doing this, then for sure they are making money.