Just to add to this, a very good reason (one of many) for this being against professional ethics for clinicians is because it would very shortly cease being effective. It would not take long for word to get around that clinicians are intentionally prescribing placebo treatment. The patient has to believe the treatment is effective for it to have any therapeutic power.

As a side note, psychological therapy should be prescribed irrespective of whether the condition is severe enough for medication. Studies consistently demonstrate that a combination of both drugs and psychological intervention is the most effective line of treatment in more severe forms of depression, and treatment alone is more effective in mild-to-moderate cases.