In the context of a proper debate, bias is expected since the opposing view will be supported by your opponent. In the context of an essay, there is no opponen

In the context of a proper debate, bias is expected since the opposing view will be supported by your opponent. In the context of an essay, there is no opponent. The author is responsible for drawing conclusions from evidence which they use to support their thesis. The corollary is an opinion piece. The author is free to derive evidence which supports their pre-determined conclusion. Such articles are able to be complete fabrications and misleading while lime-lighting in our media as legitimate articles to report. If the reader is not aware of the author's bias they may be mislead in their own conclusions. Therefore it is not ad hominem to draw into question the bias of an author of an article. Doing so may invalidate any argument they have made which relies on the intended omission of evidence which may counter their argument. Sometimes it is the only way by which a reader can distinguish an opinion piece from a proper essay when the distinction isn't made clear by the publisher of the article.