Why are airplanes parked at the gate with max rudder deflection?
The control surfaces of airliners are not connected to the pilots' control via cables; they are operated by a hydraulic pump. When the engines are shut down, there is no hydraulic pressure in the pipes, and the control surface is free to move.

The rudder is moved from the center position by wind. If you observe the gates, you should note that all airplanes have their rudder deflected to the same direction. In a perfect no-wind scenario, the rudder would stay neutral.

There are no gust locks which the pilot must arm before leaving the cockpit on Boeing aircraft (and I presume Airbus too). Gust locks are, however, common in general aviation aircraft. They can also be found on some small business jets. 

The reason being, control surfaces on small aircraft work "both ways" - if you move the elevator by hand, the yoke in the cockpit will move as well. These are called "reversible flight control systems". Therefore, when the control surfaces are moved by wind, the entire system - yoke, cables, wheels, bearings etc. are all moved back and forth. This brings a significant amount of wear and stress to the system. Strong wind may also cause the surfaces to deflect past their designed maximum, causing damage.

The control surfaces on airliners are much heavier and much stronger. Also, they are not "directly" connected to the cockpit's controls - when there is no hydraulic power, pretty much every component is disconnected. Unless you're facing gale force winds, it is safe to just let them swing free with wind and gravity.