In theory, anyone can do anything but they do have a reputation to protect.  Being the largest bitcoin exchange, why would they risk their reputation?

Money... that's the reason...

Thing that they were not audited ( were not they? ) gives a chance that this can be happening... Sitting on such big pile of cash can shift your business practice.... They can be doing it with small % of the funds at their disposal just to gain extra $... but I don't think they can do it with amounts enough to move the market....

Anyway, it's my IMHO only... and they can be clean as a child's tear ( or maybe not )