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.
Most of the people does not read the terms of agreement and even the FaQs so they don't know how the bonuses works but most of the bonuses today are more likely with the use of the more you play the higher the chance of having a bonus such as a rake back thing. For example with the blackjack.fun the highest wager have a bonus btc the same way with another platform. Also, they have the form of surveys and tasks for other gamblers to receive a bonus reward.
^ I don't know if the bonuses or promotions will be on the TOS or the FAQ which is not constantly doing their promotions.
Probably if you are looking for bonuses go to their social media account follow them and if they are here, just follow their announcement thread, that will probably give you how to find a bonus. Most gambling platforms have always this welcome bonus upon deposit and this is very common right now on each gambling platform I don't know if this thread of list in gambling casinos has been dropped here but for the community here it is very easy to find a gambling platform offered good bonuses.
https://bitcointalk.org/index.php?topic=5341966.0, faucet consider also a good advantage tough.