What restrictions are there for flying near urban areas in the United States?
This varies from country to country but here in the US the Federal Aviation Regulations define the minimum safe altitudes:


  Sec. 91.119 Minimum safe altitudes: General.
  
  Except when necessary for takeoff or landing, no person may operate an
  aircraft below the following altitudes: 
  
  (a) Anywhere. An altitude
  allowing, if a power unit fails, an emergency landing without undue
  hazard to persons or property on the surface. 
  
  (b) Over congested
  areas. Over any congested area of a city, town, or settlement, or over
  any open air assembly of persons, an altitude of 1,000 feet above the
  highest obstacle within a horizontal radius of 2,000 feet of the
  aircraft. 
  
  (c) Over other than congested areas. An altitude of 500 feet
  above the surface, except over open water or sparsely populated areas.
  In those cases, the aircraft may not be operated closer than 500 feet
  to any person, vessel, vehicle, or structure. 
  
  (d) Helicopters,
  powered parachutes, and weight-shift-control aircraft. If the
  operation is conducted without hazard to persons or property on the
  surface-- 
  
  &nbsp;&nbsp;&nbsp;&nbsp;(1) A helicopter may be operated at less than the minimums
  prescribed in paragraph (b) or (c) of this section, provided each
  person operating the helicopter complies with any routes or altitudes
  specifically prescribed for helicopters by the FAA; and 
  
  &nbsp;&nbsp;&nbsp;&nbsp;(2) A powered parachute or weight-shift-control aircraft may be operated at less than the minimums prescribed in paragraph (c) of this section.


In your case, the sea-planes can fly closer than regulated since it falls under the "Except when necessary for takeoff and landing" clause. 

These minimums apply in all air spaces however different airspaces surrounding cities (typically Class B and Class C) may prevent you from ever getting within 1000ft (or even a few miles) of the city due to clearance. For example you can't get near a city under a bravo space without ATC clearance and even then you can't fly where you want like in other airspaces.   


  but how is that and other private flying entities like news choppers


News choppers and the like still must fly by these rules. They often compensate with bigger camera lenses. The somewhat exception to this rule is generally police, fire, and rescue/medical operation choppers/aircraft. This of course is taken on a case by case basis but ATC usually gives them precedent and clears them for whatever they ask within reason. I actually met a guy that was on the council that designed the Philadelphia Bravo Airspace. The airspace is specially designed so that the medical helicopters can take off and fly under the bravo shelf from any hospital in the city. They can do this and still be a safe distance from buildings as well.


  and ad-banner planes governed?


Banner towing planes, like helicopters must also fly by these rules. Although they are not seen in cities all too often I generally see them at the beach. Although beaches are generally not defined as a congested area and generally the planes are flying over "open water" thus they must be only 500ft from the people on the beach (depending on how you interpret the law).