Because she has a gun

This morning, rushing up the subway steps, i was behind a group of cops. Weighed down by donuts and weapons, they were taking their sweet time walking up the stairs, while I was hot on their tail, trying to get to work on time. A couple steps in, one of the cops turns to me and smiles:
"Why don't you run her over?" he asks, speaking about the female cop in front of me.
"no, that's ok, i'd rather not."
"Why not?" He winks.
"Because she has a gun."