In a carrier plane drinking alcohol is not allowed except when served by the operator, regardless of the number of seats. For private aircraft there is no equivalent rule.
This is not a way to sell more alcohol at a higher price, rather a way to know how much alcohol is drunk and stop serving before aggressive or distracting behaviors develop.
§ 121.575 Alcoholic beverages.
(a) No person may drink any alcoholic beverage aboard an aircraft unless the certificate holder operating the aircraft has served that beverage to him.
The actual criteria is the beverage has to be provided by the operator. This applies to all Part 121 operators. The same rule exists for Part 135.
In addition, operators should have related procedures for cabin crew members:
C. Air Carrier Procedures. Air carriers should have adequate procedures contained in crewmember manuals and training programs outlining the specific duties of crewmembers and ground personnel regarding the use and service of alcohol. For example:
- Procedures to handle disturbances that may occur involving the service of alcoholic beverages;
- Procedures regarding the removal of a passenger who appears to be intoxicated; and
- Procedures to handle passengers who may have brought their own alcoholic beverages on board.
2. General Aviation
I was not able to find a similar restriction for Part 91. There is a restriction for carrying drunk passengers (§ 91.17.b) but this doesn't prevent them to drink alcohol when aloft, e.g. from TSB report, for a Cessna 185 accident in 2010:
It is possible the passengers’ level of intoxication contributed to their inability to
recognize the gravity of the situation and stop the interference in time for the pilot to
regain control of the aircraft before impact.
A final word from this article:
Business aircraft and charter passengers pay handsomely for their private flights, and they expect a high level of pampering and comfort in transit. Serving them fine spirits en route forces
pilots and charter operators to carefully weigh the risks, and to determine when and how to say no more.