Most of the centralized exchange also does the same thing of deposit bonus for the benefits of getting new customers as usual. The only user that can actually withdraw off their referral bonus are the celebrity or the ambassador of that company. It's kinda hard getting something good off your referral if you are a normal individual on your own.
The same thing goes to gambling sites, they are only doing that to attract as much customers they want to come play deposit and play o on their site.
You have mixed the referral bonus and the deposit bonus, and both are not the same thing; they are totally different. Withdrawal of referral bonus is not too hard as once it reaches to minimum amount of withdrawal, anyone can withdraw it, but the deposit bonus on gambling sites depends on several games and slots, and also there might be a wagering quota too.