They are artificially manipulating the market. That's about as textbook rigging as putting three masts on a ship.
If there was an obscure way to play craps that made the house advantage negative they would either change the rules or ban the practice. You could call it rigging but by that token every game is rigged, that's the whole point, you're playing in the hope of beating the odds. The rules and odds are completely transparent, 'rigging' would be like using weighted dice.