Just because someone uses the term 'disease' as an excuse to continue drinking/drugging doesn't mean it's not exactly that. They could just as easily use a traditional disease as an excuse in the same capacity.
As far as the self-inflicted argument goes, couldn't you likewise extend that to any host of diseases? How about adult-onset diabetes? More often than not it's caused or exacerbated by lifestyle. Does that make it something other than a disease?
How about skin cancer caused by sun exposure, or lung cancer caused by asbestos exposure? Should we fault people for being so stupid they dared to step outside the house without sunblock? Or crawled into their attic without having a hazmat team clear the area first?
With alcohol, you have a product that's widely used - by the majority of the population. It's shoved down your throat on billboards, in magazines, on TV. And yet - despite the deafening social voice screaming at you to drink, drink, drink... we know that a percentage of people who do (errr... 'us') will, for a variety of reasons, develop dependency issues so overwhelming that they'll literally keep the cup to their lips as their life goes down the toilet. A physical, emotional, and psychological dependency which is directly caused by the action of alcohol on the nervous system.
If that's not the definition of a disease, I don't know what is.
EDIT: Sorry bamboozle - you were typing the same thing I was at the same time!