If the author of the question does not specify an operating system, then the answers should be valid for as many operating systems as possible.
This does not necessarily imply that the potential answerer has to list all possible variations of the solution. An answer could describe a general concept or approach and mention how to apply that concept to one variation of the problem in question.
If a potential answerer does not feel capable to address the wide variety of operating systems to actually fully cover the question, then the answerer should ask for clarification through a comment.
If he answers disregarding the fact that he will not be able to fully answer the question, he gambles. If the question is phrased more specifically later on, the answer could end up being wrong or irrelevant. At that point it should be treated like any other incorrect answer.
If the answerer feels like he was treated unjustly through this procedure, he should have asked for clarification before answering.