As an outsider, the remarkable thing about this to me is that one can (for now) sell the same bitcoins twice, right? If I own 1 bitcoin pre-fork, I can sell it for $2700 and then sell the exact same bitcoin again as Bitcoin Cash for $700. Crazy.
On the other way, people who want to buy BCH can move their money to those few exchanges, and inflate the bubble, hoping in the greater fool. Ergo, the $700 prices.
Related question, is there a way to see inflow/outflow of dollars or any other real-world currency from/to these exchanges?