I agree with this completely. No-one seems to be able to demonstrate exactly the point where an optimizer becomes a spammer. It's quite natural to keep on "optimizing" to improve one's ranking. The example of alt and title tags is a good one. What is unethical about alt text which uses a whole phrase to describe what the image actually is about? ... or title text which gives a short description of where a link leads to? Where is the line to be drawn and how is anyone to know where this line is?
I used to be an architect and we had something called Building Regulations. The regulations were very clear, and didn't leave one guessing if the building would be closed down at short notice by an invisible higher authority.