Not all bum cases started out as an alcoholics although some of them falls into such category.
They might turned to alcohol after being in the streets for some time. Alcohol is a depressant that helps the body and mind to forget some temporary things.
Alcohol is still a luxury just like having money is a luxury. To give alcohol to a stranger is a blessing. Beggers don't have a choice anyway so if you give them a hot shot to warm them up, it is a blessing.