Have you gone through their rules, terms and condition to be sure that it isn't indicated or is indicated there??
If it is then you have to abide by it to be able to get your winnings..
It's a good thing anyway that you got your cash regardless, just ensure to read through websites next time so you don't get into confusion.
The site confirms about the confusion and they already processed the withdrawals, it looks like a good promotions for every user here in the forum who are interested to try their platform, the OP just got lucky to have that free money. Though, reading ToS is really important so no site will take advantage of you and of course to avoid any problem as well.