I suppose it depends upon what a person's definition of disease is. If no one agrees upon what the term means, then the conversation is pointless:
disease
Pronunciation (di-zēz′)
1.An interruption, cessation, or disorder of a body, system, or organ structure or function. Syn: illness, morbus, sickness
2.A morbid entity ordinarily characterized by two or more of the following criteria: recognized etiologic agent(s), identifiable group of signs and symptoms, or consistent anatomic alterations. See also: syndrome What is disease? Find the definition for disease at WebMD
Just because poor life choices lead to the qualifiers listed above doesn't make alcoholism or diabetes any less of a disease. It seems that most people define disease as something that's not caused through personal choices. The definition above does mention cause, but does not discount something as being classified as a disease simply because the cause can be through human behavior.
I used to completely disagree with alcoholism being classified as a disease. Now? Technically is it a disease; however, I think it would be more helpful to classify alcoholism as addiction--disease is too broad a catagory. Addiction treatment is in its infancy and it will be interesting to see the research.