Weighted dice. Go back a century and it's much easier to ban gambling using dice than it is to ensure that the games are fair.

Or it was a [poor] way to try and ban certain types of gambling while allowing others.

Same thing, really. Dice games tended to erupt into violence amid accusations of dice being weighted; there was no practical way to sure that the dice weren't weighted, but the state had an interest in preventing violence; so the solution was to ban those games while allowing other forms of gambling which were less likely to become violent.

It has to do with some quirks in the Indian Gaming Regulatory Act, and its interaction with state law. Card Craps is sometimes referred to as "California Craps" because it was invented to sidestep restrictions on the use of dice to determine the outcome of any wager in California tribal casinos. The situation is similar in other states with Indian casinos, which is why this game has spread.