What's not allowed? Drinking the beer you brought? Why would you be allowed to bring it on board but not be allowed to drink it?
You are allowed to bring it onboard but you are to consume it in your stateroom, do people bring it to public areas.... probably.... would disney reconsider the policy if they noticed everyone with a cooler bag at the pool and a drop in sales that hurt the bottom line, probably...
Another reason beyond $, is that it limits the ability for bartenders to cut off those that appear to intoxicated.
Heres the official policy
While adult Guests ages 21 and older may bring alcohol onboard, the following guidelines apply:
Alcohol brought on board may not be consumed in any lounge or public area.
Guests who arrive in the dining room with a bottle of champagne or wine that has been brought on board will be charged a corking fee of $20.00 per bottle.
Beverages must be packed in your carry-on bagwhich must not exceed 22 inches wide, 14 inches high and 9 inches deep in dimension
Coolers filled with personal items (e.g. soda, alcohol, chips, candy, etc.) may not be brought on board.