It will be too generous for casinos just to give bonuses to all its users including idle accounts. But, there's nothing to lose if you will do it, so not a bad idea at all. Some may offer deposit bonuses or wager bonuses depending on your account activity, so I wouldn't expect much if you will do this kind of trick in different casinos.
The thing is the definition of a bonus is actually quite one way for most people, they always think it's something guaranteed like money, but most casinos actually don't give out such bonuses. They instead give out free spins, free chances for x game, free tickets for y lottery, and stuff like that, and that significantly decreases the amount they have to give out since, well, they don't have to pay for any of those now do they. This'd probably only happen on big casinos though, since even if someone won it's something they can set to whatever they want prior the bet, but if they give out money, that's an insane amount for an insane number of users.