IANAL, but I think what might turn out most problematic for TheDAO in the end is not the soft fork/hard fork proposal or even the loss of trust, but their - often quoted - own terms of service[1]:
> The terms of The DAO Creation are set forth in the smart contract code existing on the Ethereum blockchain at 0xbb9bc244d798123fde783fcc1c72d3bb8c189413. Nothing in this explanation of terms or in any other document or communication may modify or add any additional obligations or guarantees beyond those set forth in The DAO’s code.
That document is the single link that connects the Ethereum toy world with the world of real, legally enforceable contracts. If any court actually takes that clause seriously and treats the DAO code as legally binding terms of service, I think that could have two consequences:
1) Intent: The terms written on the website don't say anything about crowdfunding, voting processes or the supposed purpose of the DAO. They do however say very clearly "the code is the law" and even point out that this will cause risks for the investors, should the code contain bugs.
That writing IMO makes the current argument that the attacker violated the "intent" of the contract very hard to argue.
2) Contract amendmends: As far as I know (as a layman), legal contracts may only changed of all affected parties agree to the change. Therefore, if the DAO code should be treated as a legal contract, you could interpret the soft fork as an amendment - even though not all of the DAO token holders were asked for agreement. In the most extreme case, the attacker could sue and demand that his "theft" is cashed out, even if the soft/hard fork proposal is accepted.
[1] https://daohub.org/explainer.html