Teaching is a profession. This means you are salaried to do a role. Some of which takes place in school hours and some takes evening, weekends and holidays. I doubt your beleaguered HT is going to put up with a down tools approach.
Yes, it's salaried so there's a reasonable amount of preparation done outside of directed time.
However many of the things teachers do are done because they love the students and want to go the extra mile. You say no head teacher would put up with downing tools, but as long as they are working the directed time hours and the teaching is good (which will take some reasonable prep beyond directed time), they can't really do anything about staff having breaks where they don't work, not running sport clubs, not doing evening performances, not spending their own money on classroom displays because the school didn't have the money, not buying classroom resources out their own pocket, not doing residential trips etc.
Schools, like many organisations, run on a mutual understanding of good will, and give and take.
As with any organisation, if you take more than you give and endlessly batter staff then morale is low and even the most enthusiastic workers decide to stop giving.