I don't know how much of it is down to the individual teacher, and how much is based on the "method" or curriculum that a school uses.
Some teaching styles rely heavily on homework, hard work and constant revision. Personally, I find these methods slightly outdated, and tend to be used by administrations or teachers that aren't very progressive.
A guy called Alfie Kohn recently published a book about this very subject called The Homework Myth: Why Our Kids Get Too Much of a Bad Thing.
It sparked a lot of interesting debate because he makes out that, before highschool, homework actually doesn't have any substantial benefit.
And even during highschool and higher education, the relation between homework and achievement is apparently pretty weak.
There have also been counter-arguments about this issue that are worth reading.
Teacher's assign excessive homework to students because they want them to be ready for exam related queries through revision. That is why they give homework to students. But students should take it positively and try to work hard.