Unfortunately this is the very common on most casinos. You either get a free start money to check out the website, but then you have a lot of limitions on the money. You won't be able to cash it out, its meant to be played on the site. How could any company survive while giving 50$ bucks to every new customer.

 I think deposit bonuses are a good way for first timers on a website to get to know the games and don't lose all your money at the beginning.