A few years back, there was a national law passed "No Child Left Behind (NCLB)" that has made this an interesting dynamic over here. Basically the US government tied the hundreds of millions of dollars they give to the states for education to requirements that the state adminster grade level tests that each school is expected to get better at each year, as well as improve their graduation rates each year. Regulating absences causes a major problem. On one side, the schools need the kids in class to get them to do better on the tests, on the other side, if they penalize absences too much, they lower their graduation rate. Each local area handles it differently based on what they think will work best for their schools and students.
(In reality it is much more complicated than this, but it gives you an overview)