Downloading a ROM is illegal because the law says so. Its really that simple. Since no video game is old enough to qualify as public domain and abandonware is not remotely a legal term, every video game is still covered under US copyright protection laws. Those laws very clearly state that downloading games is considered piracy and is therefore illegal.
While I'm no expert on US law, I believe it's not that simple. The only way to backup the cart is to extract the ROM and I'm fairly certain that whether I extract the ROM myself or get a third party to do it, if I own the game and therefore a license to use the game I'm doing nothing illegal.