I have to wonder what goes through the minds of any instructor who overloads students with work or who insists upon such exclusivity that it endangers the grades received from the other instructors. Doesn't that defeat the purpose of education in the first place?
In terms of workload, instructors may have a lot less power than you might think. Often, the quantity or length of assignments isn't entirely up to us. For instance, when I was teaching Introduction to English Composition while I was working on my Master's, all TAs were provided with a syllabus from the university. We were required to assign three essays (the weight and length of each was determined by the university) and plan daily in-class activities (for the participation grade). We were required to adhere to the university's policy for attendance. We were required to grade based on university rubrics. Even if the instructor is allowed to design his own class, he probably has a designated number of assessments that he must assign in order for that course to be accepted by the curriculum committee.
As for insisting on exclusivity, it's completely unreasonable in most cases. Generally, when a course is going to require lots of extra time and effort (for instance, student teaching, the thesis-writing class, an external internship), the university will recommend that students plan to take less than the maximum load. If it's a requirement that a student only take one course in a quarter, it will be made clear in the department's list of requirements for that major. An instructor can't arbitrarily decide that students only work on his class.