Are public lectures popular in USA?
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?