Maybe rules is the wrong word. Perhaps technique is better. I like the second definition from Merriam Webster (sorry - can't afford the online OED):

2a : a body of technical methods (as in a craft or in scientific research) b : a method of accomplishing a desired aim

I would say the "Rule of Thirds" is a technique that can help establishing an aestheically pleasing subject placement. What these rules do provide is a concrete way to discuss artistic elements. Suggesting the rule of thirds can be clearer than suggesting that the subject be moved down and slightly to the right.