I've worked in IT at a tribal casino, and this type of thing is fairly common. It not the exact same as far as needing to know stock holdings, but they dig very far into your debts, credit, investments, etc. As well as do extensive background checks. This is all to ensure you are not a huge risk to employ. If you're doing okay on your investments, you don't have outrageous outstanding debt, and your salary is good compared to your debt, you probably won't be as inclined to steal from them.
I'd assume this is all being done by a licensing agency? (ie, OPG, office of public gaming). Usually, you'll have to get a gaming license to work for a casino, and the licensing authority is the one that requires all of the extra information to issue you a license.
Honestly, it's typical, but annoying. Welcome to tribal life, buddy. Where they only need 20% of the information they ask for, and they'll probably lose all of it in a week anyway.