Question: Why do Crank Bros Mallets and other such "downhill" pedals advertise that you can ride them unclipped? How do you do this without accidentally clipping in?

Yeah and then they go home and clean and lube them. Playing around in a race where the clock stops once you cross the finish line is completely different from performing day in day out in the real world.

Oh no? There are many studies which confirm that pulling on the pedals produces less power for the same amount of effort. For example.
"The aim of this study was to determine the influence of different shoe-pedal interfaces and of an active pulling-up action during the upstroke phase on the pedalling technique. Eight elite cyclists (C) and seven non-cyclists (NC) performed three different bouts at 90 rev . min (-1) and 60 % of their maximal aerobic power. They pedalled with single pedals (PED), with clipless pedals (CLIP) and with a pedal force feedback (CLIPFBACK) where subjects were asked to pull up on the pedal during the upstroke. There was no significant difference for pedalling effectiveness, net mechanical efficiency (NE) and muscular activity between PED and CLIP. When compared to CLIP, CLIPFBACK resulted in a significant increase in pedalling effectiveness during upstroke (86 % for C and 57 % NC, respectively), as well as higher biceps femoris and tibialis anterior muscle activity (p < 0.001). However, NE was significantly reduced (p < 0.008) with 9 % and 3.3 % reduction for C and NC, respectively. Consequently, shoe-pedal interface (PED vs. CLIP) did not significantly influence cycling technique during submaximal exercise. However, an active pulling-up action on the pedal during upstroke increased the pedalling effectiveness, while reducing net mechanical efficiency."

You know what I think is absurd? Roadies who wouldn't know what fast was if it bit them in their pansy ass lycra shorts. A roadie friend of mine recently told me I could qualify for CAT1 just based on my average commute times in traffic. My response? "I'm not interested in playing around."

Unlike shoes which have limited space for extra insulation, sandals allow a practically unlimited increase in insulation as temperatures drop. Moreover, the adjustable straps allow you to prevent overtightness, which promotes circulation and keeps the feet warm. So you can use thicker and thicker socks combined with a goretex/waterproof outer sock to customize your footwear as the weather requires.

It works so well that I got rid of all my winter boots which pale in comparison to the warmth of shoveling snow in sandals with appropriate sock layers.