It just seems to me that if some evil company buys the patents, they're not going to respect the agreement the inventors and Twitter signed. For the agreement to cause financial damage, the inventors would have to sue the evil company that obviously has a ton of lawyers. Can you afford to sue Microsoft? I can't.
No, the inventors don't have the right to license the patents to whomever is being sued. The twitter agreement gives all rights in the patent to the assignee, just like any other assignment, but it adds on the clause that the assignee agrees to get permission from the inventor if they want to sue offensively with the patent. In the future, if the assignee (whoever it is at that time) decides to sue offensively without the permission of the inventor, the assignee can (and indeed they have the right to). The inventor would then have a cause of action for breach of contract against the assignee, but who knows what that would amount to.
In one case, evil company buys patent, and pays off everyone involved to drop this agreement. Now we're back to normal.
In another case, evil company can't get eveyrone to do that, so the transaction doesn't happen.
This seems exactly the same as saying "We won't use our patents offensively, and we won't sell them." No legal documents required.