Jeff,
Sarcasm does not become you. As far as I am concerned, people are welcomed to play by whatever rules they wish. I am just not going to give strokes or play with them unless we agree to abide by a set we recognize. I am of the "golf is to be enjoyed, not endured/life is too short" type.
Jon,
I am a big believer in the rule of law. I am an even bigger believer that rules should be transparent, relatively few in number, easy to understand and fairly applied to one and all.
To understand the rules one has to know the definitions. "Serious Breach" is not described in the Definitions section, but it is well-covered in the "Decisions On The Rules of Golf". In the PM situation, it was a non-factor, period.
I can't remember seeing the word "cheating" in the rules, and I would not personally define it narrowly as the act of violating the rules with intent. IMO, if one takes the medicine/incurs the penalty for violating a rule, that suffices- you commit the crime, you do the time. In some cases, DQ is the penalty, though it is not the intent of the rules to kick people out of the competition for all but the most serious violations which are, for the most part, spelled out.
As to giving the player the benefit of the doubt, that is not true in all cases, and never without doing due diligence. In your example of the player/marker disagreement, the committee would ask other players in the group about their view of the discrepancies, the caddies, and even bystanders. Typically, a player is to report his score to the marker upon completing each hole. If a marker fails to jot down the scores until some time later, the committee could very well side with the player in the absence of corroborating evidence.
Question: let's say you are the superintendent at Carnoustie and Scotland has joined the no-inputs agreement being pursued in the EU. The Open is coming up in X months and the only way to kill off some organisms which are eating away the greens is to spray with a banned chemical. The cost of said chemical is nominal. The fine for violating the agreement is likewise, say, £1000. Do you "cheat", spray and pay the fine? Or do you abide by the agreement at the potential cost of hundreds of thousands of pounds, personal disrepute, and ruthless criticism of the R & A? BTW, there is a fairly well-known case study taught at Harvard Business School (and Ohio State where I went to school) which covers this issue (I think it was the Chiquita Bananas case). Do you intentionally violate a law when the consequences/fines are less onerous than abiding by it?