They also do not want a uniformed crewmember to be seen consuming or buying alcohol - it gives a bad image to the airline plus the fact that the public does not know that you are not on your way to a flight. Always be aware of your alcohol intake. Watch your time and don't take the risk of being caught with alcohol in your system. You never know when the airline, in accordance to the FAA , will give you a random alcohol test before or after a flight. Because it's a Department of Transportation violation if found illegally intoxicated, it will not only effect your employment with your current airline but with any other job in the future that involves the Department of Transportation.
Drugs are also something to be aware of.
Not only are random alcohol tests conducted but also drug tests can be given at any time. Anything that hampers your ability to perform your job duties as a crewmember is a no-no. Remember, your prime responsiblity as a flight attendant is the safety of your passengers. If you have a problem with alcohol or other substances - get help. Many companies offer an assistance program to their employees.