Very weird, I remember those scamming attempt I forget the actual scenario but the same with depositing before you can withdraw thing.
That's very unethical why ask for deposit if the account is under investigation? no sense at all just an attempt to lure the player, what if you deposited and then they've claim that you are cheating their system?
Another amount that you'll going to chased and it's more annoying.
In OP's case though the investigation is already over and on the screenshot they've already said the conclusion which is to close the account but they're giving second chances. I know it's unusual for casinos to do these but I was also given a second chance and I remember the others that posted a complaint in the main thread also got flagged for multi accounting got released as well.
You're not wrong, there are scam sites that does the same thing where their site would trick you into thinking that you win and they'll require you to deposit before you could withdraw the funds. But like i've said in my other posts, most gambling sites does the same if you're a new player(assuming OP is a new player based on the userid) that participated in their promotion for the first time and win you'll be asked for a deposit since that's how most casino promotions work.