This is covered in 18 USC 331. It says that whoever fraudulently alters coins is guilty of a crime. The key word there is "fraudulently." If you alter a coin with the intent of defrauding somebody, you're guilty of a crime. If you alter or destroy a coin you own for reasons other than committing fraud, then you're just wasting money.
Now a days those machines actually just store your coins and give you the pre made crushed "penny". My local zoo charges a quarter and gives you the thing
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