Borrowing a document from other people to fulfil the KYC requirement is not a good option but some people still use that to get the bonus from casino. They don't think much about the risk that they can get when casino know what they do. Maybe that will not happen in short run but that will cause a problem in the long run. But those people will not care because as long as they can get the benefit from casino, they will still do that and even they may reopen more accounts and use the other people's document to chase what they want. That is why the cheat gamblers will always found the way to stay at the casino and get the benefits of that before the casino found them.
Everyone will never run out of ideas to get what they want, like those who want to get more bonuses, of course they will outsmart it by using other people's documents if they themselves have used their documents and succeeded in getting the bonus. I myself have experienced something like this where my friend borrowed my personal documents to get a bonus and if successful he said he would share it evenly. Although actually it is the wrong and risky way because our private documents are used, and actually we can refuse our friends to borrow our documents because there is no obligation in this case, I myself before lending it consider it first, for example everything will be safe and my friend will be responsible if something happens then I lend it and vice versa.