As a former tenured professor (not fired, I resigned to go work on my startup) this is not true. Yes it is nearly impossible to fire you directly, but what the universities do is play musical chairs. Every couple of years (sometimes every year) there is a reorganisation where there are fewer positions than tenured professors. Guess who gets cut when this happens.
That's the point: the majority of college classes in the US today are not taught by tenured or tenure track faculty, but rather by adjunct faculty who are paid by the course rather than an annual salary. My wife used to teach five classes per year as an adjunct and made under $20,000 for it, and she had zero job security from one semester to the next. (Sometimes her contract for a class wouldn't show up until a week before it was due to start.) Tenured professors at the same college teach six classes per year, and make... rather a lot more. (We've got expectations for scholarship and service, too, so some additional pay is reasonable, but that would have to be more than half the job to make that ratio reasonable, and it's definitely not.)
Universities rarely grant tenure today, compared to decades past. It is security-- if you can get it, and very few do anymore. Adjuncts have very little, if any, security in their role.