Academic freedom is not a legal concept, and unfortunately the law trumps the ideals in this case. However, academic freedom isn't exactly unlimited: a tenured physics professor who doesn't teach a class on physics, publish a paper on physics, or advise a student for 10 years will likely be fired at most institutions (unless they have a nobel prize or something). Professors working at state schools have additional limits on their academic freedom. This is why you don't tend to see theology departments at state schools.

Also, in practice, you could reasonably argue that there generally isn't much academic freedom in most fields, since most research is grant-funded, and that is controlled by (usually) governments.