Some schools require midterm dates on the syllabus, some don't. But in this case, the schools are more likely imposing structure on the professor rather than the other way around.
If you know exactly what you are going to teach for a semester, you can plan it all out. But god forbid if the class gets stuck on some material...if the schedule is strict the prof will just move on and let the curve sort it out. I preferred more flexible teachers in college and there were real reasons why universities in the US are high regarded.