Indeed we can draw the parallel to law school. In the U.S., a
Juris Doctor degree is considered a professional postgraduate degree. There is no corresponding degree program at the baccalaureate level. First-year law students are a smorgasbord of prior studies, almost none of which are directly relevant to the study of law.
Really? you might say. Yes, while degrees in political science, history, administration, and so forth might introduce topics in law, the program of law school relies upon almost none of that. What qualifies you for success in law school is the ability to think logically on your feet and to read and recall large tracts of text. The facts you bring with you from the previous degree are largely immaterial.
Further, if one proposes to study intellectual property law, one's bachelor's degree
must be in the field that pertains to the kind of intellectual property in question. Therefore someone coming to law school to practice patent law must have something like an engineering degree. But under no circumstances would that person be an "engineering postgraduate." No, those would be my students who were continuing their education
in engineering, not applying what they've previously learned to some new and different field.