The reason for this is to warn others when one person scams several people all at once and to prevent others from being scammed in the near future. For example if someone who is trusted were to scam 5 people all at once then they would potentially still have a net positive trust feedback  if all negatives were weighed equally, on the other hand under the current system scamming 5 people all at roughly the same time will result in a score of roughly 600 less then their previous trust score (pre scam).