I doubt it's a courtesy. Yes, they can give you a ticket for 1 MPH over the limit, but they don't because they all know that there is a good chance it'll be thrown out in court. 1 MPH is a minor difference and can easily be thwarted by the argument of "is your equipment calibrated and guaranteed to be accurate to such an extreme precision?". No technology really is, so they don't bother. They usually give 5 MPH because that's a good number to be over without their radar guns or whatever being questioned.
Locally the police chief said there was no quota system but he would discipline any officer that did not write at least X (I believe 120) tickets a month, and they would be citing for 1 mph over....
That was 2 years ago N they stopped the 1 mph thing, I have a feeling that it did not hold up well in court....
"The sword dose not cause the murder, and the maker of the sword dose not bear sin" Rabbi Solomon ben Isaac 11th century
"Don't be so open minded that your brains fall out!" Father John Corapi.