Why do people cross their arms during encounters with the police?

cuz they need a hug...

It is a sign of defensive or building a barrier between themselves and the situation/person. 

As a sign of defiance. They become the line to not cross.

I didn't know they did.  It's a sign of closing yourself off from whatever is going on.  Probably best not to.  I never cross my arms.  Or legs.  I consciously stopped crossing my arms because I don't like how it looks.  I stopped crossing my legs when I had my knees replaced.

It is cold out.