Casino sites aren't obliged to provide a signup bonus there are just sites that do it to attract players. It's their own discretion and a part of their own strategy. If you're the kind of player that sign up just for bonuses, you'll surely have a hard time withdrawing because most of them will require you to deposit a beginning amount to cash out your winnings. They all have their own TOS.
True. This is promoted by the website owners in such a way that the players are not interested in spending time reading TOS when they join the website. So they offer different bonuses for marketing their website, which actually attracts crypto gamblers to that particular website. But since the user does not follow the TOS properly, when he tries to withdraw and does not succeed, he will realize that such marketing offer is more effective for whom.