There are transaction costs for exchanges but there are also transaction costs for miners. Coinbase I think just lumps these into a single number, but other places you can choose the amount that goes to miners, on the risk of taking longer to confirm your transaction if you pledge a low amount.
So? We're talking about an index by an exchange. I seriously doubt it that they'd actually transact on the blockchain; and if they would, that's just plain stupid, because it only increases costs without any benefit for the investors (who still don't actually own their coins).
Well if you own the exchange the effective transaction costs are 0.