It is illegal to download roms/iso's/gcm's from the internet and around here it is a touchy subject, so if you want to play Gamecube games on your computer you will have to search google for guides on how to rip your own games.
Yup. But they don't waste their time with small-time downloaders, they only go for the big fish to release and distribute it... that's with games anyway, I'm not saying anything about the RIAA.
Just buy your games, there's nothing more satisfying than having a nice shiny little disc, a beautifully pristine mint manual and the satisfaction of being able to do whatever you want with it - rip it, light it on fire, trample it, auction it...