They almost have to own up to things like this. Liability for one, if they knew about a security breach where passwords were compromised and did not tell anyone, that could make them liable. The Target CEO lost his job because he sat on news of their breach for a couple of months. Also if you know you have to give out bad news, it is better to take the bull by the horns and initiate it, rather than scramble around after the fact trying to explain both what happened and why they did not tell people their security was compromised.
In the world of public perception, the first press release sets the tone. While bad news, at least Sony has more control over the dissemination rather than trying to come up with a way to explain it.
Finally, because they are a publicly traded company, there could also be SEC reasons as to having to own up to bad news that could potentially affect the stock price.