You can't make money performing a play that someone else wrote unless you get permission!
You can't project a DVD of a movie onto a big screen and make money from people watching it unless you get permission!
You can't record yourself reading someone else's book and sell the recording or charging companies to put ads on the broadcast, without permission!
You can't show people the full plot and details of a whole video game that you didn't publish without the publisher's permission!
Just because doing that on Youtube has become a trendy thing with it's own cute name (Let's Play!) doesn't mean it's not IP infringement. What Nintendo is doing is saying "We respect our fans and are letting them slide, and even letting them get paid real money making clearly infringing derivative works, we just want a 50% cut of the money because we are the ones who made and published this game which is the basis of the over half (generously) of the video's content. You get as much credit for adding your personal touch to the broadcast/recording, as we do for making the whole game, 50/50 which is more than fair." Also for the record, in the new Mario Kart, Nintendo DOES also allow people to record and edit highlight videos of their races and post them directly to Youtube through a free app. Acting like they are this anti-social bully is completely against their corporate philosophy of gaming being fun and inclusive for everyone.
Most public domain software is free, at least at first glance.