1. You can absolutely be fired for cause as a tenured teacher in California, and it happens all the time. It's certainly more difficult than most other professions, but there is no law or policy preventing teachers from being fired.
2. Teacher pay is not entirely based on seniority. Graduate education, certifications such as the National Board of Teachers (which is merit based and does require observation) or a bilingual certification, coaching a team/leading a club, all can contribute significantly to a teacher's salary.
3.After a 10 years of service, a teacher with a masters degree in my area makes $63K/year. Adjusting to a 12 month schedule, they'd be paid $75K year. With only a BA that drops to $54K. Hard to argue they are overpaid at that rate IMO.
It's extremely easy to see the excessively high rates that some regents or college administrators make, and attempt to use that anecdata to draw a full conclusion, but in my experience the rank and file make on average 25-30% less than they could in private industry.