Imagine if you were a hardware store employee, and someone in the store asked you what would be more useful to break a window, a brick or a sledge hammer -- you'd have no way to know why they wanted the information. You'd just answer them, and that would be the right thing to do -- unless they told you why they wanted it.
Nice analogy, but I don't think it fits the situation. A better one would be someone asking a pharmacist how to make poison. Perhaps the person asking is a researcher studying antidotes. If so, that person would already have some knowledge about poisons. In this case the person asking had little or no knowledge of the dangerous field he was asking about. Should knowledge about poisons be protected by the first amendment, sure. Should people with that knowledge share it when appropriate (assisted suicide?), that's an individual choice based on an individualistic definition of "appropriate". Should they share it with anyone who asks regardless of how much it looks like the person is intending to do harm to self or others with no indication there is a valid reason behind it? Nope.