Depends where you are. In most countries, it is totally illegal to deface currency. In others, it is illegal to deface them up to the point where they can no longer be used as currency.
The short answer is, it isn't. The long answer is, under 18 U.S.C. 331, "Whoever fraudulently alters, defaces, mutilates, impairs, diminishes, falsifies, scales or lightens any of the coins coined at...
No. If you own the coin, it is your loss if it is destroyed. The idea that it is illegal to deface coins goes back (at least) to the Roman Empire. Money belonged to the Emporer and the people were...
It is illegal to shred, tear, or mar or otherwise deface any US currency. This includes melting coins. An exception is made for penny smashers. In addition, no one would probably be too bent out of...