A possibly legal (or illegal) response?
You (we) are dealing with an unregulated exchange, and most of the members know about that status, so talking about a legal response is a bit odd, IMO.
MtGox can do whatever they want, and their customers can ether stay (like I am) or leave.
no matter how many times people repeat that, it's just untrue. mt. gox might wish to exist in an unregulated anarchist utopia, but it's subject at least to the laws of japan and very likely to other laws that apply under treaty obligations. at the very minimum, customers of mt. gox have contractual rights against it, and contractual rights very definitely exist even in the absence of express contractual terms; usually gaps in contracts are filled in with implied terms based on trade usage and practice.
you're right that one option a dissatisfied customer has is to leave. but if the customer is not just dissatisfied but wronged, another option is to sue. i'm not an expert on japanese contract law, but if the exchange existed in the united kingdom or the united states (two legal systems with which i have experience, even though not a lawyer), a legal claim by someone whose profits were disgorged as part of a 'rollback' would, in a situation like this, very likely be weighty. at a minimum, it would not easily be dismissed as frivolous or obviously wrongful.
again, in case it's not clear, a 'rollback' here would be identical to the following situation: someone steals 500,000 btc and transfers it into mt. gox. they sell at low prices, and someone buys a number of bitcoins at $4. subsequently, the exchange rate rises to $13. mt. gox decides, however, that the trades are illegitimate, reverses them, and keeps the spread for itself. or, it turns over the money voluntarily to the authorities, or to the party it believes was the 'rightful' owner of the 500,000, in its own legal or ethical judgement.
in all those cases, what mt. gox would have done is very literally 'theft', and users would almost certainly have legal claims against it. i cannot distinguish that case from this one, substantively. surely mt. gox's position is no better merely because it, or one of its customers, was the party that was stolen from. its response is equally wrongful and self-serving in this case as in the ones above; it just happens to be allocating its portion of the illicitly recovered 'profit' from other people's trades back to one of its chosen customers.