Some companies are choosy when permitting the authors of case study and hence may decline the permission. Hence authors may develop cases based on published data.
If the data comes from sources that are in the public domain and do not require any specialized clearance to access, then I see no reason why permission is needed. If any of the data comes from material not typically available to the public (e.g. internal communications or documents), then permission might be ethically responsible.
On the other hand, permission may be needed from the original copyright holders of the information published in the secondary sources (even if these are in the public domain). They may or may not be in a position to grant that permission -- it depends on the conditions under which they were allowed to publish the stuff in the first place.
first, do you need permission according to the law, national ethics comittees, etc? that will, of course differ between countries. but most 'western' countries would say no you don't: it's public knowledge.
second, and as Dash pointed out, you might need permission from the original copyright holders. But that also depends on how you use the material (extensive citations, using charts, images, etc).
third, demands from the journal you are publishing in. Some journals want to safe-gurad themselves and require you to get permission although you legally/ethically are not obliged to have it.
But in general, I would say that you do not need permission from the company as long as the data can be considered publicaly available and you have no reason to believe that it is a complete falsification or slander.
I have noticed that there is no consensus as to what should be followed as there are some practical issues like the authenticity of the sources, accuracy of the data and the facts, and names of the persons involved, particularly if there are conflicts and legal issues.
No it is not. These cases are referred to as Library Cases. While developing such cases though, every source has to be carefully cited. It is a good practice to let the company know that a case has been written on their organization but you do not need permission.