The note did not say Dear MR. Policeman, as you posted and did your Google search. However, still a good point. Looking at what you said, it's as if the sniper is presenting an argument to the authorities. Dear Policeman: (you may think you are God, but) **I** Am God.
There's no evidence of it. See yesterday's article:
"Physical evidence has been scant, with one taunting clue apparently left by the shooter near where a 13-year-old victim was shot - a tarot "death" card with the words "dear Mr Policeman, I am God" scrawled on the back. "
Besides, the same people said the same thing about "Dear Policeman". The original letter said: "Dear Peace Officer"