The roms are not illegal if you own the cartridge
Depends on how you acquire them.
If you own a backup cartridge, and backup your own specific game, then you are entitled to a backup copy of your original cartridge.
But any ROM downloaded off the internet is illegal, for a # of reasons. First and most simple is, if you take a cartridge and back it up, you've now created the 1 copy you are legally entitled to. If you then take that copy and upload it to a webserver, you've now created two copies of the original, which violates the DMCA.
In a nutshell, any ROMS on the internet are technically illegal, whether you own the original game or not. Technically.