Related to why TAMU leaving is different than talking to Arky or Neb leaving, the legal issue here is "tortious interference", which basically is interference by a third party in a contractual relationship that results in a breach of that contract and harm to the litigating party.
When Nebraska and Colorado left, the Big 12 did not dissolve and their media rights deals (one of which was expiring) were completely unaffected. Thus, there is no harm and hence no legal case.
Again, contacting Arky did not affect their contractual status as they did not leave the SEC due to those contacts, hence no legal case.
TAMU leaving the Big 12 may or may not kill the conference, but it will definitely breach the Billion dollar media rights deal signed by the Big 12 schools this past year. And clearly that is a harm for every school, to the tune of over $100M per school, not the kind of damages that can be papered over.
The lawsuit threat is real, and this situation is different than the Nebraska/Arky ones. Furthermore, Iowa State is doing the right thing here by protecting their interests, and not waiving their right to sue - especially because they would have a good case.