Do you know why rhetoric is called a public activity?

Check this definition.

http://www.merriam-webster.com/dictionary/rhetoric

What do you think?

I think rhetoric is called public activity because it is used in everyday activities in life publicly by everyone and anyone.

Yes. Rhetoric is the art/skill of communicating with others.

Ok thank you. Also is the goal of rhetoric to attract the audience by advertising or displaying something?

Right.

Ok thank you very much :)

You're very welcome.