You're missing the point. A police officer is sworn to uphold the laws of the state of . . . The laws on moving violations are state laws in any given town in any given state. They're not local to one town or another. So how do we swear the cop to enforce the laws of the Commonwealth of Pennsylvania and then tell him that he can only enforce them in this area? His oath and those laws are just as valid in any other part of the state.
While I have enjoyed travelling at a speed greater than the posted limit on many occasions, and I got away with it (mostly), the New Jersey argument is right. They're doing what we swore them to do.