The person whose account was hacked (if it was one person) is entitled to their Bitcoins and it's MtGox's responsibility to provide that to him. Kevin did nothing wrong making a large purchase of bitcoins, but now that he KNOWs the bitcoins were stolen, he's possessing stolen property, which can be a felony. The fact that MtGox's account was hacked makes all the difference here, as it makes the bitcoins which were traded stolen property.
Nobody here knows that's what happened though.
And there are many ways this could be interpreted in wholly different ways. One way could be, depending on what actually happened, is that MtGox's system sold me bitcoins that they didn't have a seller for, so they sold them to me themselves.
That's a bit of a stretch, but saying what happened here is positively "theft" when there's no evidence of any "theft" happening, and it's hard to say that even if what they say happened did happen, that it's actually theft or not.