Is it ethical for advisors to automatically coauthor papers?
No, automatic co-authorship is not ethical – irrespectively of the discipline! 

The following is from the "DFG Proposals for Safeguarding Good Scientific Practice" (pp.82f, emphasis added), which, for instance, strictly forbids "automatic" co-authorships and gives a couple of common negative examples:


  Authors of an original scientific publication shall be all those, and only those, who have made significant contributions to the conception of studies or experiments, to the generation, analysis and interpretation of the data, and to preparing the manuscript, and who have consented to its publication, thereby assuming responsibility for it. [...]
  Therefore, the following contributions on their own are not sufficient to justify authorship:
  
  
  merely organisational responsibility for obtaining the funds for the research,
  providing standard investigation material,
  the training of staff in standard methods,
  merely technical work on data collection,
  merely technical support, such as only providing equipment or experimental animals,
  regularly providing datasets only,
  only reading the manuscript without substantial contributions to its content,
  directing an institution or working unit in which the publication originates
  
  
  Help of this kind can be acknowledged in footnotes or in the foreword.


In fact, DFG (the German Research Council) requires you to sign strict adherence to this ethical code on each and every grant proposal. Other funding agencies have similar regulations.