How? Lol basically they have control over whatever's in their site. You balances, your account information, the games that you play, name it they have access to do anything they want as long as you're in their platform.
Probably they have it but if they will took the information of your account then that is consider as breaching and a violation. Yet, they will not do it because betting platform wants to make sure that your account details will be safe.
As much as you will be using blockchain then you are safe. There is no way that the betting platform could access your account or digital wallet. This is why I will agree to OP regarding on the use of blockchain rather than entrusting digital currency to any websites.
But, in terms of how you play the betting platform does control.over it and that is why many of the gamblers having fun playing in betting and at the same time winming over it.