Correct, bonus abusers will find a way to get free money out the casino and prompting the administration of the service to lock accounts, halt withdrawals, massively ask for KYC. etc.
It could even devolve into a case which affects people who did not even claim bonuses or even worse the closure of the casino.
Casinos are businesses and no business benefits from giving money away. So it is fair there are some requirements to claim those bonuses or take advantage of them.
They are running a business and not a charity which it would be understandable that they would really be putting up some terms and conditions before you could make yourself able to pull off some withdrawal or not but most of the time due to the requirement then it wont really be something simple or easy for you to do so, and this is why lots of people are really not that interested to deal off with bonuses.
Completely free money do sounds unrealistic which pertains or do talks about having no terms and can be simple pull it out, it cant just happen.
Not a charity but a business that will work for them if ever they will find the right way to market the business, I see your point and it's true casino owners will not just give you that promise bonuses without doing anything, it's their way to attract new users and they will do everything to allure people to use the platforms, it will serve as give and take, they will let you use the bonuses and try some luck, in hope that once a gambler enjoy the game, they will come back and deposit more funds.
A marketing strategy that always has corresponding benefits for the business, else, they will just give money for nothing and it's not the goal of each bonus but to make it as a working tool to bring more users around the house.