I reckon it's more to ensure that none of the GC contenders are disadvantaged by being trapped at the back of the peloton while someone makes an attack. It's such a short stage, it is going to be brutal racing from the moment the flag goes down, and if you're not near the front of the pack at the start, you'll be out of contention very quickly.
Last year's 100km stage to Foix was insane, as was the 109km stage to Alpe d'Huez in 2011. This one is even shorter, and could be even more exciting to watch.