The other side of the very same coin is when you award top X% people for performance. When you fix the reward/punishment pool, people have much less incentive to work together, because every help you give to someone else is reducing your own chance of success.
I'm not sure why people set up such incentive structures. Is it greed? Or some kind of stupid belief that a little bit of competition will help? Yes, if you let every good performer get a bonus regardless of how many others are also getting it you'll have to pay out a lot of bonuses, because you'll have a lot of good performers. But isn't it exactly what you want to have?