When you stop to think about it, many words (including fetish) are co-opted from one definition and context and then turned into rhetorical weapons to insult, degrade, persecute, and ridicule. (As to whether and/or how this phenomenon might manifest itself in a positive application, well, I'd have to think about that.)
Clearly, the French conservatives in the context you cite are using the word fetish derisively. Their use of the word may have been in fact a legitimate assessment of German scholarship, but it could just as easily be a shortcut of sorts which obviates their using a more intelligent, rational, and dispassionate argument. Call it a rhetorical shortcut.
Sociologists (such as Durkheim, perhaps) and other experts in scholarly pursuits are prone at times to overreach their area of expertise, intruding on the research and findings of scholars within other disciplines. Ideally, all the experts should pool their findings, based on common ground, and come up with a version of reality which incorporates all legitimate perspectives and findings. Some polymaths (of whom there are few, I believe) are able to do so; your "average" scholar, however, begins to think he or she has a corner on truth, which is clearly risible. Who knows, maybe at one time, so-called educational institutions of liberal arts strove to do this. But I diverge.
Now there may be a time when the use an inflammatory word might be apt, if only to get people's attention. The responsible rhetor goes on, however, to expand upon why he or she chose to use such a word, giving reasons, instances, definitions, and examples/illustrations of why the inflammatory word is apt in a given situation.
A word which is used similarly today is the word paranoid. At one time, a person afflicted with paranoia was assumed to be suffering from a severe mental illness. Nowadays, when a person evinces the least bit of defensiveness, an observer is bound to say, "There's no need to get all paranoid, Bill [or Betty]!"
Sociologists, anthropologists, and sexologists used to define a fetish as an abnormal preoccupation with something which is clearly asexual, such as objects made of leather or rubber (e.g., shoes, as in a "shoe fetish"). Another name for this phenomenon might be paraphilia, which is essentially a sexual preoccupation with an asexual substitute for person-to-person sexual relations.
In conclusion, in the marketplace of ideas, when an outspoken opponent wants to belittle his or her opposition quickly and easily, they'll either co-opt a belittling word or phrase by taking it out of context, or engaging in the ol' ad hominem, the logical fallacy of name calling! Either "strategy" requires little or no intelligence and is used unethically when it is not accompanied by proof.