Basically, they're giving a little bit of money to uploaders, and those uploaders advertise their site everywhere they can, they spam big forums, they beg for invitations on private torrent trackers, etc. So they could have the "0day" content.

Uploaders are responsible for the success of the file hosts, if new file hosts weren't paying no one would use them. Their only plan for success is sharing a little bit of the revenue with uploaders.

Better do not trust file hosts, they're here for the profit, and there are a lot of cases where these new file hosts delete account of an uploader just for the lulz. And obviously, they won't miss that one uploader at all, as they got bunch of new uploaders who will advertise their site everywhere...