You mean if they really wanted to steal players money why did they only steal some and not all of it?

Because that's what scam casinos do.  1xbit is the best example.

They keep all of their losing players deposits + all of their winning players winnings. 

Players are less likely to create a public issue if the deposit is returned. 

Returning players deposits and confiscating their winnings should always be a red flag - there are very few cases where doing so is the appropriate decision for a casino.  A players balance belongs to the player.  It doesn't matter how much they deposited.

Please do not ever give a casino a pass because they returned a deposit and only kept player winnings.