Why is JsonRequestBehavior needed?

Why is Json Request Behavior needed?

If I want to restrict the HttpGet requests to my action I can decorate the action with the [HttpPost] attribute

Example:

[HttpPost]
public JsonResult Foo()
{
    return Json("Secrets");
}

// Instead of:
public JsonResult Foo()
{
    return Json("Secrets", JsonRequestBehavior.AllowGet);
}

Why isn’t [HttpPost]sufficient?
Why the framework “bugs” us with the JsonRequestBehavior.AllowGet for every JsonResult that we have. If I want to deny get requests I’ll add the HttpPost attribute.