I think it has alot to do with the issue. The military accepts resident aliens as volunteers to fight for our Repulic's freedom. I would use the same standard for the police forces, also subject to background checks as in the military.
There are significant restrictions on resident alien enlistees.
“I would use the same standard for the police forces, also subject to background checks as in the military.”
The same requirements as required for a commissioned officer should apply to law enforcement.