A deposit bonus is not a free gift to be precise, you get the bonus when you make a deposit using your own money, and that doesn't make it free. A free gift means something that you receive for doing nothing and spending no money from your own pocket which is not the case here, bonuses are a token of appreciation by the casinos to you but it comes with some requirements so that it is not abused.
Deposit bonus mightn't be a gift to you because you feel you have to do something to get it but we have other bonuses that can be classified as a gift because your money isn't involved in getting those bonus like the welcome bonuses that casino gives to new users.
When you receive deposit bonuses, it can also be classified as gifts or reward for completing a tasks which says the same thing to me. You're not buying the bonus with your money so why aren't you concerning it a gift, this could also be a sign of entitlement.
All bonuses could have been free but we know they'll be abused by those that just want to make free money without putting in effort by betting. The casinos are only putting this requirement to prevent them from paying unethical people and going bankrupt.