How to explain to a student that it is common to include a supervisor as a co-author? In my field of research, it is common to include a supervisor as a co-author of a student's PhD research related papers. A supervisor mainly contributes by helping to improve the writing of a paper, such as emphasising its contribution or clarifying the content, etc. Which means that a supervisor does not contribute anything directly related to the research carried out in a paper, for instance, a supervisor may not even understand a method used in a paper.

Based on all the answers of this question, it is clear this custom is not applied to all fields, and I am not trying to argue if it is correct to do so or not.

I want to know how a supervisor explains to a new PhD student that he/she should include a supervisor's name in his/her papers. What happens if a student refuses to follow a custom?

PS: when I started my PhD, I already knew about this custom, so my supervisor did not need to explain it to me. But how about those that don't know or know differently?
If the student truly is writing a paper independently of the advisor, then the advisor shouldn't be a coauthor and it would be inappropriate for the advisor to demand this.    

In the more usual situation in which the advisor provides the the dissertation  topic and works directly with the student in developing the dissertation (holding regular meetings, reviewing drafts of the thesis, meeting with the dissertation committee) and then papers based on the dissertation are published, the participation of the advisor would normally merit coauthorship.  In these situations the authorship of papers should be settled before the research work begins.  

I make it a point to explain this to students before starting work with them on their theses and dissertations.  I simply wouldn't agree to supervise a student who would not agree to sharing authorship in this way.