Fantasy worlds rarely take into consideration the full ramifications of the effect of magic in the game world. Its probably easiest to take the approach that magic is rare enough (player-run magic users notwithstanding) that the effect is minimal on the fictional world that it might still resemble our own.
But would it? Imagine a few situations which were actually somewhat real problems in eras where superstition and fear of witchcraft were more prevalent:
#1 You are a farmer. Your livelihood in fact, your very survival is dependent upon your crop yield. Suddenly, half your crops sicken and die of some kind of disease, which has left your neighbors (so far) untouched. Your family may starve as a result. You know witches who have the power to do exactly this. You suspect a few people in your village of having that power. And one of them has a grudge against you.