Police Helicoptors-automatic right of way?
Not knowing the full details of where & when - I am unable to give a reason as to why this happened. However, due to certain situations (again, not knowing the full facts of the incident the Police were attending), there can be inmplmented a "No Fly Zone".
These sorts of restrictions are normally in place for a major incident, a VIP movement or if it is believed that the presence of another aircraft will hinder the operation. As you can imagine, there are not that many times this "restriction" is used, but it has been known.
Hope this goes some way of clearing up the matter.
Mac