Tenure is not "older than the country itself." The tenure system, as we know it, began in the German Gymnasiums and was not adopted in American universities until the 20th century as part of a large university reform movement - the same reform movement that led to standardized tests like the SAT and ACT.
In theory, tenure is supposed to protect free inquiry. In practice, it merely ensures that junior faculty research conforms to the conventions of their field. In the case of MLA-dominated humanities departments, this means hewing to the shabby orthodoxies of the campus Left.
In a nutshell, here's how it works: at the top US research institutions, a junior faculty's tenure case depends 80% on his/her success in publishing in peer reviewed journals. In humanities, those peers are predominantly drawn from the fascist PoMo/Semiotics crowd. Therefore if you're in CritLit or journalism, conservative views = heresy = no pubs = no tenure = living in a van down by the river.
Of course, tenure is only sacrosanct to the Left when it is convenient for them. Consider the case of Joe Horn and Lino Graglia (tenured UTexas professors who spoke out against Affirmative Action), or the UT administration's crusade to make faculty oppose the Hopwood decision.