Mt.Gox doesn't owe anyone the ability to sell BTC for fiat cash. If someone bought BTC then that's their decision to invest in that asset.
Mt. Gox doesn't have to offer the ability to sell BTC for fiat cash.
The problem is that they do offer that ability.
Then they keep your money.That's usually called "theft".
No, a bank keeps your money too. It's only theft when they won't give your money back.
As I said Mt.Gox can give anyone their money in the form of bitcoins instantly. Nobody has refuted that. They can probably give anyone their fiat money within the same day who claims it locally (Japan); nobody has refuted that either. They have been shown to give people their money back, though after some delay, via the original wire request.
That is far from theft.