Andrew, the relevant certification standards in the FAA are as follows:
For FAR Part 23:
Sec. 23.671
General.
(a) Each control must operate easily, smoothly, and positively enough to allow proper performance of its functions.
(b) Controls must be arranged and identified to provide for convenience in operation and to prevent the possibility of confusion and subsequent inadvertent operation.
For FAR Part 25:
Sec. 25.671
General.
(a) Each control and control system must operate with the ease, smoothness, and positiveness appropriate to its function.
(b) Each element of each flight control system must be designed, or distinctively and permanently marked, to minimize the probability of incorrect assembly that could result in the malfunctioning of the system.
[ (c) The airplane must be shown by analysis, tests, or both, to be capable of continued safe flight and landing after any of the following failures or jamming in the flight control system and surfaces (including trim, lift, drag, and feel systems), within the normal flight envelope, without requiring exceptional piloting skill or strength. Probable malfunctions must have only minor effects on control system operation and must be capable of being readily counteracted by the pilot.
(1) Any single failure, excluding jamming (for example, disconnection or failure of mechanical elements, or structural failure of hydraulic components, such as actuators, control spool housing, and valves).
(2) Any combination of failures not shown to be extremely improbable, excluding jamming (for example, dual electrical or hydraulic system failures, or any single failure in combination with any probable hydraulic or electrical failure).
(3) Any jam in a control position normally encountered during takeoff, climb, cruise, normal turns, descent, and landing unless the jam is shown to be extremely improbable, or can be alleviated. A runaway of a flight control to an adverse position and jam must be accounted for if such runaway and subsequent jamming is not extremely improbable.
(d) The airplane must be designed so that it is controllable if all engines fail. Compliance with this requirement may be shown by analysis where that method has been shown to be reliable.]
You can see the differences, and more rigor of the Part 25, which is for large aircraft.
For my limited experience with large aircraft, there is either a pin that either pilot can pull, to allow the two pilot's control wheels to operate independently (DC-8, if I recall from 30 years ago), or something purposefully breaks in the control run with high pilot force (B707), or there is a calibrated release joint like a spring loaded cam (Dash 8).
Light aircraft generally will not tolerate the weight or cost of such systems. The risk is just not perceived as being great enough to warrant such complexity for these aircraft.
Think of it a different way: The certifiers know that us little plane pilots are much more likely to make the many stupid mistakes which Big Pistons points out, so a control jam is a long way away from being a "top of the list" risk for little planes. The risk does not warrant the cost and weight for the system to protect. The bigger planes, being flown by two competent pilots, reduce the likelihood of those basic stupid mistakes, so the other risks come more to the forefront. Plus, there are now the two pilots, so twice the chance that someone can fly at least half the plane, if the jammed half can be released. It's now worth requiring the system, in an aircraft where the weight and cost can be carried.
In a Dash 8 aileron system, the gizmo looks like this. it turns the cable motion into a pushrod motion, with the cam release in between the two. Can you imagine the cost of this assembly in a Cessna?
Sorry, Photobucket refuses to resize the photo to 640 by 480, even when I take it in "small".
