Well, it should be like this, don't you think? Bonuses are there to make people "taste" the game, and then, if they like it, they must deposit to play more. Although, it's not exactly like that. Firstly, you can play in demo mode for free as much as you want. Secondly, there are exceptions. I myself won a withdrawable amount couple of times on various sites, starting with just bonus money.
Yeah, a bonus is simply to give a bit of a boost to a player.
If it was something were people actually could withdraw the whole amount without playing and putting some money in, then a lot of people would simply take the money out, making it not work for the casino at all.
The thing is that if people can do it, they will do it.