If everyone must *assume* crap like this is going on, then how could a single exchange take place? Next time you try to buy a coffee from your local BTC coffee house, what's to keep them from taking your BTC and not giving you a coffee? In the end, we all must have trust.
If the coffee house refuses to give you the coffee, you can call the cops etc.. That is what keeps the frequency of coffee scams to a fairly tolerable low level in real life. Exchange owners have no such deterrent. They can do, without risk of punishment, everything that
lawyers lawmakers and regulators have found necessary to prohibit to stock exchange owners, over the last 100 years. Why should't they do it?
EDIT: wrong word