Comma Logic Fallacies

Why do different authorities of the English language disagree about the palcement-or logic, for that matter-of commas within sentences?
How are genuine, concientious persons meant to self-teach about 'correct usage', for example, when comtemplating whether to include 'The Oxford Comma', or whether to break long compound predicates with commas?
  Here's an example sentence which explores that second point: "A policy is a statement of intent, and is implemented as a procedure or protocol."
  When costructing this sentence, its obvious that the writer felt that the insertion of the comma is natural. Indeed, it marks an obvious intonation queue, allowing the reader to parse the sentence better. But... It's also a blatant grammatical error, right? By grammatical ruling, the comma shouldn't ever function to break compound predicates, or sentences which appear to have an ellipted subject in the second 'clause'.     
The comma functions as though the second verb phrase, 'is implemented', contains its own subject, but it doesn't: the sentence has one written subject, 'A policy', and two verb phrases. Why are eminent writers breaking this rule everywhere I look?
Can anyone on this forum offer up some advice, particularly on that second expanded point?
Why are there inconsitencies in comma rules still reigning through the language? I've been trying to master the comma for months now, and these inconsisencies arent making it any easier for me.

Thanks in advance for anyone's input.