As evidenced by this thread, it could easily happen that the player read the druid description, came to a different understanding of the rule or in-world taboo, and agreed to follow what they believe to be the correct interpretation. It might not agree with the DMs interpretation.
When I rolled up a druid, I talked with my DM about metal armor and alternative material medium armors, but only because I spend an inordinate amount of time reading about this crap on the internet. If I came back after having that discussion and said I do something that we had agreed was out of bounds, then I would be a jerk. But conversely, if there was no common understanding beforehand, and the player had assumed they'd be able to use medium armor, and the DM made that impossible after the game had begun, then I think maybe the DM is being a jerk.