Why are tenured academics against academic freedom? For/ced Faculty Development

This will be a post about my utopian proposal allowing academics with tenure to move as easily as possible into other fields of study.  I am talking only about Literature Departments.  In my experience, tenure means death for 80 percent or more of the profession.  You just keep on what you've been doing and try to police "the field"and how other faculty in your Department teach what they teach.  Everyone is supposed to stay were the were when they were hired.  Some people train themselves and move from one field (say, Joyce) to another (say American lit). But the notion of fields remains intact.  What if the American system were to become more like the German? In Germany, you dissertation and you Habilitationsschrift have to be in two different fields on completely different topics.  When I say "more like," I mean that that tenured faculty could be encouraged to train themselves in new areas of study, including already established fields. After writing a second book, you would get a year off and, in addition to your raise, a 15-20k grant to develop two different,  new upper division courses in fields other than those you have already taught and published on.  All you would have to do is produce a research bibliography, attend the annal conference in the field, and teach the new courses.  The assumption is that even advanced faculty (in rank and in years of age) need to keep developing and deserve support (time off and cash) as much as do untenured faculty.  Freedom is inseparable from force.