Are public lectures popular in USA? 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.
Public lectures are not generally required, but are one of the well-understood benefits of university education.  For most American universities, lots of interesting speakers come through and are widely advertised.  It's good for the university because it shows it's a place that such speakers find worthy of visiting, and for the faculty by giving them opportunities to interact and collaborate.

As for the students: if world-class speakers are showing up and talking about interesting things, why wouldn't students choose to attend the lectures that interest them?