What is not OK is buying a big lump of those coins in private from the exchange without the clients' knowledge.
Is fractional reserve banking actually illegal, especially with Bitcoins?
Banks are quite happy to use it and screw us .
Technically that would not be fractional reserve banking, or even banking at all. While the price remained constant, the exchange would still have on its hands enough capital to honor all the clients' account balances -- except that it would temporarily be in the form of stocks or other investments, rather than bitcoins.
There is no law that requires an exchange to store clients' dollars as dollars and bitcoins as bitcoins, unless that is stated in the user agreement. Converting coins to dollars would have been a terrible risk, but they may have thought that it was OK.