The article explains why it makes sense. The institutional investors' trades don't match up that often. If you allow HFT's then they will buy/sell on other exchanges (or in other dark pools) that match the other side of your (barclays in this example) dark pool's institutional investors trades.
You don't charge the HFT firms since they allow you to charge your other investors.
You don't charge the HFT firms since they allow you to charge your other investors.