Because the main symptom of back pain is the patient reporting "My back hurts". If the patient instead reported "I have this lump in my testicles that wasn't there last week", I'd certainly think it's likely he might have cancer, and some good medical advice would be "go and ask a GP about that".

Back pain is a symptom in itself. Maybe a disk has been shifted, or it's caused by a kidney infection. Maybe it's just muscle pain or sciatica related. Hell, for all we know, the back pain is caused by the chair in question. Without proper examination you can't rule these things out and make a professional recommendation as to what to do about it other than "go see an expert or general practitioner".

By phrasing the recommendation the way he did he made sure that it is understood that a) he is treating the patient for an unrelated issue, b) he isn't in a position to diagnose back related problems and c) he never examined the patient's back problems.