These gambling sites wants to entice their customers to make deposits, that is why they'll give them bonus promises, and the gambler will be excited to claim it, not knowing that there's a price before they can use the bonus. It's a lesson for us to always scrutinize anything that is a giveaway, whether it's favorable to us or not.
Very well said, yes the gamblers get enticed by the gambling platform's offers and in excitement they deposit huge amount of money in hope to earn free bonus by completing wagering requirements but they don't know that most casinos do that to trap new users and they can change their TOS without any notifications even within days. As far as I know, there's always a price to pay if someone wants to earn money from those free bonus and the ones who understand that will always avoid bonuses.