I thought it was odd that they were smart enough to exploit the bot, but not smart enough to retrieve the money. Most people would have tried to withdraw a bit by bit to prevent alarms from being raised.
He might have not realized the extent of the fake volume. Could have assumed he was a vanishingly small fish whose tiny withdrawal would not be noticed in the noise of an exchange reportedly doing $100MM daily.
Perhaps Bertyar88 didn't believe they were breaking any rules, and fully expected to be able to withdraw the funds, regardless of what anyone knew.
I guess they never thought this would work so wildly well. They would have expected for the site admins to tweak the bot to prevent exploitation.