That's the teacher's union, not tenure. Tenure is a really rare thing in public education. College is different, and tenure serves a specific, important function: it removes the threat to the career of a professor who is teaching concepts or ideas that are socially or politically unpopular. It makes it okay to express concepts that push the boundaries of a given field. Without tenure, questioning the findings of a time honored textbook might jeopardize your career if the dean happens to be that book's author. While it can also protect some bad apples, the concept of tenure in college is pretty vital.
Teacher's unions in high school are a bit different, and it's really more a matter of A: red tape making it difficult, and B: administrations who lack the backbone to take action. Frankly, most admins don't give a shit if a teacher is yelling at students. They probably got yelled at as kids, and assume it's good for them. There's a LOT of "I went through it, so now you're going through it too" at every level. It's what takes good, ambitious, student-driven teachers and beats all of the optimism out of them.