In a comment to my answer to What would be the side effects on a Druid of wearing metal armor?, @Kevin asked:
"So, RAW a druid will never choose to wear metal armor or use metal shields, so the main thrust of the question is based on a flawed premise." – If the Druid is a PC, is it appropriate for the DM to say "Your character would not do that" if this is attempted?
Which got me thinking that if I said "Your character would not do that"; am I trampling on their agency?
The original question is about D&D 5e. In that ruleset, RAW is that druids "will not" use metal armor or shields and there is no provision for what happens if they do (like there was in earlier editions). The issue is not about the consequences of breaking the rule; it is about if the rule can be broken.
In a more general sense, it is about dealing with a player who wants to break a "rule" (not just about druids and metal) on the basis that it is a) not physically impossible and b) carries no RAW sanctions.