Aside from what Mneme has already said, the 24 hour situation isn't true, either.

It's not legal to download or distribute roms in any way shape or form. If you were to create the rom yourself, from your own cartridge, then it's okay for you to have, though not distribute.

The 24 hour a-okay is really just something someone made up to feel better about what they were doing. It's not any more or less illegal if you own it for 10 minutes, or 10 years.