aroa - playing devils advocate here.
Where do you draw the line? At some point flying for commercial gain goes from being a favour for a mate to part of a business and hence a cost of operation. If you leave a loophole to fly for business as long as the pilot is not paid for it, you can bet someone will exploit that - "He is was not a pilot, your honour, he was a fish handler with a PPL". Maybe the flying is no different, and there is nothing to stop a PPL being suitably qualified, but a CPL sets a higher benchmark expected for someone operating an aircraft day in, day out, in line with a commercial operation.
We have already seen organisations that push pilots to skirt bad weather or fly overloaded, stretch maintenance to the limit or adopt dodgy practices just to save a buck, and these are organisations primarily concerned with aviation. What would happen if any business could just buy a plane and use it with just 40 hours of flying experience, no oversight, no check on procedures, no systems to ensure they were not cutting safety for an extra buck. Some oversight is better than none.
I don't disagree with you - at the low adhoc end a flight with a commercial purpose is no less safe than any other, but what is the transition point from helping a mate to a commercial operation?