Read the ISU faculty handbook. No faculty member is getting fired for failing to perform. The timelines are ridiculous. It takes long periods of sustained poor performance before disciplinary action with any teeth even begin. Those timelines also far exceed the typical tenure of a Department Chair, so they aren't going to bother. It takes gross misconduct."Lifetime employment" simply means that we do not have to re-apply for our jobs every year, as we do while we are on the tenure-track. How many people in the private sector have to re-apply for their jobs every year?
Once tenured, a professor is not "hired for life." Any tenured professor can be fired for a variety of reasons, just like in the private sector. Tenure is a much welcomed security blanket that further guarantees academic freedom, but it is not an absolute guarantee of employment.