It's within Nintendo's right to demand a part of the revenue but it's also incredibly stupid. Unless you're at the very very top of the pile you earn very little through youtube as it is.
Taking 40% of the revenue of someone who is advertising your product for free is ridiculous, there's a reason other companies haven't gone down this root.
Nintendo are just trying to grab money with this move and i've always viewed them as a pro-consumer company who are passionate about games and care about what they create first and foremost. This move has really thrown modern-nintendo into question for me. Are they not that company anymore?
Also Nintendo send the money to you in USD through PayPal, this means in countries with other currencies you have to pay associated fee's ontop of the 40% nintendo are taking.
