[harimau777]

"Labor" isn't being used in the sense of just "someone who does work". It's being used in the sense of one of the sides in class warfare. Police generally work on the side of management rather than the side of labor therefore they aren't labor in the sense being spoken about.

Note that solidarity is an important principle in the labor movement. So the fact that police the police might act as "labor" in their personal negotiations with management isn't considered to make them part of "labor" in the general sense; because they don't show solidarity with other unions.

Think of it in terms of regular warfare. Just because two nations separately fight the same country in different wars doesn't make them allies. They are only allies if they support each other in wars against that other country.