This might sound pretty strange, considering nutmeg is something you might have in your cabinet now. The explanation makes it pretty clear, though.
Nutmeg is the seed of a large evergreen tree native to the Spice Islands in Indonesia. For a long time, nutmeg has been in high use in the Middle East and around the world. In the US, it is used mostly for flavoring in cooking, and rarely for its second use- medicine.
In Saudi Arabia, its use as medicine means it must be classified as an official drug. Its medicinal properties are disputed, but what is undisputable fact is that large quantities of nutmeg can be used to produce hallucinations and headaches.
Consuming too much nutmeg can actually lead to death. Therefore, it kind of makes sense that it would be illegal.