Police do not patrol neighborhoods like that....those are special circumstances such as high risk search warrants......
For the life of me, I cannot envision ANY event where police have to look like infantry. Riots? Civil Unrest? Call out the National Guard.
One problem (of many) with this militarization of the oilce is like what's happened with SWAT Teams. "When all you have is a hammer, everything looks like a nail." Hence we have thme kicking in doors for white collar crimes, or worse yet, shooting dogs and people when attacking the wrong house.