Why do they say I am committing a fallacy when I am just insulting someone?

On the internet, one is often accused of committing the so-called 'ad hominem'-fallacy, which, according to Wikipedia, is defined as a fallacious argumentative strategy whereby genuine discussion of the topic at hand is avoided by instead attacking the character, motive, or other attribute of the person making the argument, or persons associated with the argument, rather than attacking the substance of the argument itself I find this a bit strange. No, when I — say — call somebody "an idiot", I am not necessarily doing it as part of some clever "argumentative strategy" with the ultimate end goal of changing the topic or otherwise diminishing the arguments offered with irrelevant distracting comments. No, I am calling them an idiot because that's... just my bloody opinion. And opinions are what one shares (along with arguments) in a discussion. So why do people accuse me of committing a logical fallacy? In fact, are they not the ones committing a non-sequitur fallacy, as it does not follow from me making an insult that the insult was specifically made in a strategic attempt to discredit their argument? Basically, my question is, why do people assume that any insult is an ad-hominem fallacy, when, according to the definition, only certain insults are ad-hominem fallacies. The two examples below should demonstrate this distinction. It is fairly obvious which one is a fallacy and which one is an opinion. Discussion A Person A: Smoking is healthy because my doctor says so. Person B: You're a bloody idiot. Discussion B Person A: Smoking is unhealthy because my doctor says so. Person B: You're a bloody idiot. Therefore, we can safely assume your argument to be wrong. Clearly, only person B in discussion B is committing the ad-hominem fallacy. The other person B is just giving their opinion. And yet, you will find that most people on the internet will describe both responses as ad-hominems. Why is this? Is my definition of the fallacy wrong? What's going on here?