They had to reimburse their users or they would have ended up losing their reputation. They would have lost most of their users. Offering reimbursement made current traders feel more safe. They now think if the exchange ever gets hacked, their funds will still be returned (i know... won't always happen. But this is what they believe now).
Anyway, most of the well known exchanges would have reimbursed their users. All of them contingency reserve funds for situations like this.
We have been hearing about hacks like this for years, yet people still store their coins in the exchange.