They assumed that every download was by someone who would otherwise have paid over 53 USD for it, which by itself is an absurd delusion.
You're wrong, your presumption assume one single person to download a single file he/she never ear about.
What's most likely, people download ROMs for nostalgia, ie: something they, or their parents, bought them when they were children. So, if we assume someone download their "childhood library" which was already paid, of about ~30 cartridge (admitted the download is the right one, and didn't required multiple download attempt); in the view of the FBI, that single person "stole" 3180 USD he/she paid ~20 years ago.
You're not just supposed to lose the things you bought, you're supposed to be fined (for attempt to play the product you already paid) with price updated to current industry standard.