Actually I said exchange sparks belong to the exchange not to the depositors unless the exchange chooses to do otherwise.
... because you cannot force them to do otherwise (read: the "right" thing) 
Actually as founder I deem the right thing for the exchange to keep them. I want the exchange to have financial incentive to list sparks so they can potentially sell them. I want them to have a stake in the success. I want people to keep funds off of exchanges.
The 'right thing' for an Exchange to do would be to distribute to PTS depositors.
The financial incentives for an Exchange to list coins is to make commissions off of trades, deposits and withdrawals... and retaining custom.
Any Sparks they retain will be a dividend earned off the back of PTS depositors - not a good look for any Exchange.
Whilst no-one can compel the Exchanges to honour snapshots, I'm surprised to see a founder of a coin positively advocating that Exchanges keep any snapshot of funds deposited in them. The success of Sparkles will be based on people investing in them and widespread adoption, not exchanges listing them.
Also, if Exchanges are invested the 'Bitshares ecosystem,' they do so by proxy. They are provide a service to the people who buy and sell on their Exchanges. If people don't trade, the Exchanges have no interest in the coins and delist them. Coins should look to appeal to the people that will use them, not the institutions that only exist to serve people's needs.