I would think it's both reasons - can't monitor and cash cow. However to negate the first one, why do they give you a mini-fridge that has booze in it? They can't watch you drink that. Sorry - that one doesn't hold a lot of water - er booze.
The bottom line is the money. They very seldom cut anyone off for being too drunk. How many times have you seen or read about drunks.
Money, money, money, money.