Sometimes a GM has to nerf characters and players because they can see a loophole that may be exploited. The GM is simply taking control of the situation and balancing out the field so they can see everyone enjoy an even experience.

The delivery of that message however is what can change a passionate GM to a tyrant, and vice versa. Most of the time they want the best but frustrations from players or even themselves when missing details, can in fact impact their perception from others.