The toe is behind only because it is the last point of contact with the blocks or the track. The next frame would show the toe coming up and leading the foot through the stride. It is ridiculous to compare a sprinters start to a swimmers and say that a sprinter should only use one leg, why use only half of the power available to you?
The toe is SCRAPING. This is interesting stuff. Even if you push off with both legs, the preponderance of force comes from the back leg and back block. In every film you see the second stride much shorter, and with planting of the foot usually off to the side. Nevertheless, do it whatever way you want. Either way, no one gets DQ'd. On the swimmer start analogy you are correct. Among other things, a swimmer starts with both arms thrust forward. I have no I idea why swimming was both into this issue. Weird.
I agree it is more of a byproduct of good sprint form e.g. low knee lift rather than a skill taught. However it is still part of something which needs to be taught and not something that comes naturally to an athlete. Emphasis is more pushing off the front block and pulling the back leg through rather than pushing off the back block and landing on the front foot.