Students reasonably need to understand what is expected of them in a course. Educators need to make clear what is acceptable and unacceptable student engagement with a course. The syllabus is the natural place for this to happen, as long as both students and educators recognize it for what it is.
Students shouldn’t approach the syllabus as the maximum they’ll do… education is about expanding your horizon! The syllabus is the absolute minimum you should expect to do; the engaged and interested student will use it as a lower bound, not an upper bound.