I think the answer is on their "How it Works" page
Bitcoin exchanges sell Bitcoin. BitInstant does not sell Bitcion. Instead, BitInstant assists exchanges and other merchants in receiving payment from Customers in the most efficient way known.
in other words BitInstant gets cash through their various merchant accounts which it then transfers to exchanges "in the most efficient way known", thus undoubtedly using it's own reserves of Bitcoins which it sells on the exchanges buying your Exchange-Account a "cash deposit".
So, when the insanely too volatile, unregulated, grossly unstable, unreliably priced Bitcoin Derivative market crashes in 10 minutes time, they loose a boatload of money being forced to execute quick exchanges to transfer converted sums, the same way merchants foolish enough to use Bitcoins or Exchanges foolish enough to keep their segregated customer cash accounts in Bitcoins would have been in the massive devaluation crash.
Price stability is the critical issue that will kill Bitcoin if exchanges can't be properly disciplined into the well regulation of BTC value.
https://bitcointalk.org/index.php?topic=175708.msg1832923#msg1832923 Re: stable bitcoin pricing...The "Bubbles with Bitcoins" nightmare