The word "policeman" in itself is not strange. It was teh total phrase "Dear Mr. Policeman" that I found either juvenile or one with English as a second language. After I made that post, I read another post that had a google search on the phrase. It turns out that "Dear Mr. Policeman" was a text used in Singapore to teach grade school children!
"To educate our students on crime prevention, a series of crime prevention textbooks called "Dear Mr Policeman" were brought into the school curriculums in 1986 to teach students from upper primary to lower secondary levels the importance of crime prevention. This series, which is still currently being taught, will soon be replaced in 1999 by an enhanced series called "Safe and Secure:"