Sandal technology has been around for thousands of years and has the distinction of being mankind's original footwear. The SPD (Shimano Pedaling Dynamics) variety is a relatively new adaptation of the age old hot weather shoe that has been protecting people's soles since before recorded history. When you stop and think about it a cycling specific sandal for riding in warm weather makes perfect sense.
A stiff sole provides a sturdy platform from which to mash the pedals and the open front and sides provide plenty of airflow for cooling ventilation. Riding a bike like any form of exercise generates heat but also a useful byproduct called motivation. Why not take advantage of all that air movement to help keep the dogs cool and comfortable. A removable center section on the bottom of the tread allows access to mount a SPD type cleat.
Back in the 1990's I jumped on the clipless pedal bandwagon with everybody else but as the years went by I began to rely on the cleat-pedal interface systems less and less. I still use SPD pedals for mountain biking and fast rides on my road bike but the vast majority of my commuting, recumbent and general fun rides take place on regular platform pedals. With that in mind it's the stiffness of the sandal's cycling specific foot bed that is it's greatest attribute.
I wear flip flops and casual sandals for eight months out of the year and even during winter I wear my sandals with cotton and wool blend socks. It's only mildly annoying when a little snow gets in and makes my sock wet but it dries quickly enough. In my mind it's only natural that I be wearing sandals while out enjoying the warmer months on my bike.