In Poland where I study, public lectures by professors from other universities are not common (maybe one or two lectures every semester in my faculty). I stayed one semester in USA as an exchange student. There were 2-3 public lectures every week in our department. It was surprisingly for me as many people even undergraduate students attend each lecture. More surprising, there were active discussions after each lectures, students asking questions and even follow the lecturer to nearby cafe to talk more about the topic. In Poland, a few people attend a public lecture. Some leave during lecture and some after lecture. A few remains for discussion and almost no one asks question.
Is it the regular practice in America that students actively attend public lectures as much as (if not more) than their classes? Even most courses I attended had a guest speaker in a session.