Some ground crews turn off the seat power as they clean the planes. Sometimes, FAs turn off the seat power before boarding begins. Technically, I think that the FAA allows use of the power ports ONLY when the airplane is above 10,000 ft (i.e. between the post-takeoff chime(s) and the "prepare for landing" announcement). In practice, most of the time the FAs forget and just leave the power on at all times.

As a practical matter, I do NOT recommend that people plug a cell phone into the seat power to charge as soon as they get on the plane. You might notice that as the plane switches from ground power to engine-generated power, all electrical systems go off-line briefly. While I'm sure all circuits are protected for safety reasons, I still would be inclined to worry that sensitive equipment could be damaged if connected to the seat power outlets during the engine power-on/off processes.