Not exactly your case but I noticed this in Vegas, too, for people that are under-age. My first experience in Vegas was at age 6 when my dad snuck me on the floor and let me crank the one-armed bandit slot and I won $30.00 on a 30 cent bet on dime slot machine. Years later I went back at age 19 and they never ID'd me the entire time I bought-in and played. However, when I went to cash out my *winnings*, they ID'd me knowing I was under-age (I have a baby face). Of course not being able to produce an ID I was stuck.
So what they were doing was allowing under-age people to play and LOSE money, but when they WIN, preventing them from cashing out.
Years later I went back at age 22 and whooped some casino arse and proudly showed my ID at cash out.
** AS FOR YOUR INDIAN CASINO **, it's just another technique they use to try and prevent cashing you out. They don't like winners, especially winners that invaded their territory!