Why are the police drilling with the military? Because the police are becoming more militarised. Ever seen video of US soldiers in Iraq? The police in both our countries have been gradually getting more paranoid, even as crime rates rapidly go down, more concerned for their own safety and less for ours. Hence police special forces, hence automatic rifles, hence police tanks, hence no-knock warrants, hence tasers and pepper spray and no oversight and no prosecution for assault. And army protects and serves someone else than the people where it stays.
And why are the army drilling with the police? Because the government wants them to be good at enforcing the law if and when they decide for them to.