My suggested rule is:
A supervisor should be a co-author in a paper taken from a student's thesis if heshe has contributed substantially to the research
A supervisor should not be a co-author in a paper taken from a student's thesis if heshe 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.)