My suggested rule is:
1) A supervisor should be a co-author in a paper taken from a student's thesis if he she has contributed substantially to the research
2) A supervisor should not be a co-author in a paper taken from a student's thesis if he she has not contributed substantially to the research
There are different styles of supervisions among different people. (And also for the same supervisor in case of different students.) A style where the supervisor and the student conduct joint research is certainly legitimate and it has various advantages. In such a case it is more likely that there will be a joint paper written. (But also in such cases, if the supervisor's contribution to the actual research is not substantial then the supervisor should not be a coauthor.)

