For sure that's gonna happen since they cannot sustain their site for having free bets option to their site users. So provably what they are doing right now is part of their marketing techniques since its easy to spread out the site if there's free money involve as pot prize. Also with those experience we encounter about those things like we need to deposit first before we can withdraw our winnings then we need to be more careful with that since we don't know yet their reputation so exert more effort to research if they are running legitimate operation before doing such actions.
Please note that any site, that ask its users to deposit any amount of money before the user can withdraw he's or her winnings, is likely a scam.
This an old strategy by scammers where they first give you a substantial amount of money for free, or for doing very simple tasks, but upon withdrawal of such money, you are asked to make a deposit first, know if that they money is not money, it's just numbers on your dashboard, it can be wiped out any second or minute, when you fall, and deposit the money requested, be rest assured your money is gone.
I have encountered several of such sites, and non of them have ever been legit.