I always got the impression that it stems from the innate implication that criticism ultimately means lack of perfection. This is of course derived from ideals, and not ever attainable (perfection, that is), and so all things can be subject to a critique of said product's focus, successes and flaws. Occasionally the critique itself is not complete, sharing little or no positive illuminations nor discussing the products' strengths or innovations, instead focusing on the perceived flaws/ failures, and vice-versa(extolling the product's apparent virtues without acknowledging it's perceived failings).
From there it can break down into several different misunderstandings, like the criticism always implying or equaling dislike or hatred of the product; implications that others who enjoy the product criticized are somewhat lacking in some manner/ need to be educated or shown that the product is flawed; that criticism is a push to have the product changed or redone, or outright banned, or a perceived shaming those that do partake in the product because they revel in the failings, making them inherently 'wrong'. I'm sure there could be other misunderstandings/failings/etc.
A good example is that I liked Prometheus, and most people on that alone think I'm a moron.
Okay, not a good example at all. I'm just bitter that people demand that THAT movie makes solid scientific sense, when a racoon can save the universe and gamma radiation creates rage-fuelled superheroes and not just cancer.
Damn it, I was trying to stay impartial...