It's really quite simple. If you leave your bitcoins with Coinbase when the fork happens, they're going to keep all the Bitcoin Cash for themselves 
That's what confuses me: Coinbase insists that they will NOT keep your BCC for themselves. As such, what will happen to it?
Coinbase also claims that they may change their position on BCC, depending on how the market evolves. Thus, if BCC rises significantly, I'm also wondering whether they will wind up paying BTC holders the BCC that was due them. Will this even be doable?
(BTW, it's not entirely trivial to move BTC off of Coinbase at this point: Between the "delays" they're reporting with BTC withdrawals, other exchanges not supporting my location, and the hassle of setting up private wallets, I don't trust in my Coinbase BTC getting to me in time, or even at all.)