And/or they want to maintain a working relationship with said vendor. Going nuclear is a good way of getting _exactly_ the minimum level of service that your SLA specifies.
It doesn't stop you from getting sued, though, it merely stops you from losing. It's pretty reasonable to want to avoid a lawsuit you're absolutely certain you could win.
Vendors very seldom sue customers for publicly saying their product is defective. The negative publicity tends to backfire. Legal action can backfire even worse. If the vendor claims the product isn't defective, they have to prove that in court to win a libel action. That means discovery and examination of the company's internal documents and the complaints of other customers, all on the record.