Granted there really are some bad seeds out there. Aside from them, I just find the concepts of formal suspensions or expulsions for preschoolers, as a general rule, a bit over the top.
Preschools (and honestly, probably most schools) expel students because they reach a point where they can't risk harm to the building/staff/other students. It sounds over the top, but what else can schools do?
There is something missing in the life of a child who repeatedly resorts to acts of violence against others, and it isn't the place of the school/staff/other children to bear the brunt. I'm not talking about one or two times, or hitting a child who "started it". Children who are repeatedly violent in school need to be removed.
No one thinks that this helps the child being expelled, but having them in school, harming others, isn't fair to everyone else.
Keep the chair-throwing, spitting, hitting child, and you're going to have lawsuits, no students, and no teachers.