Because only one distinct CPID with payments and magnitude can actually solve a block, therefore rewarding one miner a disproportionate share would be unfair for everyone else.  If anyone could always solve a block it would be fair to let the floodgates swing wide open, but think about what is happening in this scenario:

User 1 has 100 machines - And pulls 90% of the block reward
User 2-11 have 1 machine each - and pull 1% of block reward each

Now in reality over 100 blocks, user 1 is only allowed to solve up to 13 of those blocks Max (because of the distinction rules), so he pulled in 9% of the reward yet we paid him 90%?  LOL, not a good idea....

The CPID rules were put in to make it easy to solo mine, so the pool has to have new rules to make it fair.

I might go in and actually track the shares that "count" by each miner (IE let them throw whatever power they have at it) but that requires a more elaborate share system, one that verifies each packet is actually going toward real work in the pool.  This would look something like this:  If big dog A has 100 machines, and he recently solved a block with his CPID, his shares dont count for 5 more blocks after (so none are rewarded to his miners).  Then I suppose we could open up the floodgates wide if we had that rule in place...  Licht should also work on this rule, because the little miners are being hosed if not.