There's only so many coins and they need to actually have them (or the cooperation of gox to be able to sell coins they don't have).
If all the major exchanges join a
voting pool there's no way for them to sell coins they don't have without being immediately detected.
This would be great, and a huge advantage to the network and for security of exchanges and customers alike.
But will the exchanges do this?
It may be they already sell BTC they don't have. These exchanges are not run by paragons of virtue!
Getting BTC-e, Stamp and Gox all doing this looks tough - UNLESS they are more afraid of security issues than being found out for what they are (potentially) hiding from the market. It is possible on balance this is the case.
It would be a leap forward if it worked - but it will take the will and a lot of work.
EDITED to remove thoughts on how they could sell coins they don't actually have - no need to freak everyone out!