What makes you think these exchanges hold the Bitcoin they say they have in user accounts?
I suspect the truth is somewhere in between. The "fat finger" was a SQL entry that credited each account with 2000 BTC each, of which some clever users immediately took those offsite and drained the very insufficient BTC balance that the exchange actually had.
In this case they definitely had an issue, haha. But in general, I'm not sure that exchanges hold full reserves. Bitstamp however is compliance-oriented, which is why we learned about it in the first place, I guess.
I suspect the truth is somewhere in between. The "fat finger" was a SQL entry that credited each account with 2000 BTC each, of which some clever users immediately took those offsite and drained the very insufficient BTC balance that the exchange actually had.