Are traditional snowshoes for either foot? Why are only your toes harnessed to the snowshoe but not the back of the foot?

There is not a left and right snowshoe. The heel is not held down because you can't walk normally without your heel leaving the surface.

Without snowshoes on, try walking ten steps without your heel leaving the floor until the ball of your foot does.

Think of the flat plane of your foot. How high would you have to lift your foot to get the back of the snowshoe to clear the snow. Walking in snowshoes requires you change how you walk, toes first.