I'm guessing neither you nor Porceliamone have ever run 100m or 200m competitively, or any other distance for that matter, at an athletics meet?
I have many times and you just don't give it everything you've got in the heats.
Hence why most of the athletes slow down - just watch any qualifying heat from the last 10 days and almost all of the top finishers slow approaching the line.
When you're expanding maximum effort and energy is when you're most likely to get injured; a torn muscle for example, or pulled hamstring. There is no point in risking an injury for a race you've already won.
This is not show boating - the dictionary definition of that is very clear.