What's the purpose of "over G" penalties in Red Bull Air Race? This post mentions that if a Red Bull Air Race pilot exceeds 10G acceleration limit for longer than 0.6s on their plane then they are unconditionally disqualified with "did not finish" result no matter how they perform otherwise.

What's the purpose of this restriction? Is this to prevent the airplane from falling apart or pilot getting unconscious during the (low altitude) race?
It's a safety rule to protect the limits of the airframe and the pilot.

See the video (also on youtube) from this Red Bull Air Race page which explains the rules. The G-limit is discussed from 3:30.

The following is a quote from the relevant section of the video.


  Maximum load factor, that is how much G's is the pilot pulling. That has
  issues [...] the structure of the
  airplane is designed to withstand a certain amount of G. We need to
  ascertain, to check, that the pilot isn't exceeding that amount of G,
  so we monitor that very carefully. A thousand times a second we're
  monitoring the G inputs that the pilot is putting on the airplane. So
  if you exceed the maximum G's then DNF (Did Not Finish)