I think that a "biological basis" means something different to you than to me. How exactly do these studies show that depression is different from being unhappy? Did they study the brains of merely unhappy people and find no such biological differences? Again, in my understanding psychiatrists and researchers make no attempt to differentiate between depression and unhappiness because they have no reason to make this distinction.
I'm going to assume that your argument is that if depression is caused by biological factors then it must be treated differently from unhappiness that is caused by life events. But in that case you will have to explain how this is consistent with the fact that "...depressive episodes are strongly correlated with adverse events..."[0]. This fact seems more consistent with either reverse causation (depression causes biological changes) or that depression can have multiple causes, and can occur in the absence of a biological cause.
I'm going to assume that your argument is that if depression is caused by biological factors then it must be treated differently from unhappiness that is caused by life events. But in that case you will have to explain how this is consistent with the fact that "...depressive episodes are strongly correlated with adverse events..."[0]. This fact seems more consistent with either reverse causation (depression causes biological changes) or that depression can have multiple causes, and can occur in the absence of a biological cause.
[0] https://en.wikipedia.org/wiki/Major_depressive_disorder#Psyc...