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.
It's not that they are generous but they are doing this intentionally to lure the people to visit them and play on their casino. Eventually, people will get hook and use their own money especially if they are winning on their early tries. Casinos can always recover what they have spent on their promotion because most of the players are going to lose.
We players think we have nothing to lose. At first maybe but there is, later on. Not only money but our personal details too. If our goal is only to earn money then I wouldn't advised such trick but it will be better if we will just look for a real job. This is the only way to earn a stable income plus there is no risk of identity theft here.