Just because the coins are legal tender does not mean that they are "in circulation" and people are spending them at stores.
Yes, the Canadian Gold Maple Leaf is legal tender, as is the American Gold Eagle. All this means is that if you owe a debt of $50 to someone, you can give them one of these coins, and they are required to accept it as payment in full.
However, the coins have an intrinsic value that is much higher than the face value. They have this high intrinsic value because of the gold content. It would be foolish to give them to someone in exchange for $50 worth of goods.
To answer your question, they could be melted down and have the gold harvested (illegally). However, doing so would decrease the value somewhat. The gold content contributes to the value, but they also have value as collectors items. The coin form also helps specify the purity and weight of the coin, and if the coin is melted, the resulting gold lump would need to be tested by a buyer to verify the purity and weight. As a result, these coins don't get melted, and people needing gold for utility purposes don't buy coins and melt them, as it would be more expensive than just obtaining raw gold.