Those examples(Covergirl and radio hosts) aren't really comparable. Youtube right now is in a state of 'wild west' in comparison to radio and commercials/advertising deals.
For example...let's say XxXMinecraftProXxX does minecraft and minecraft like videos. And he usually tends to focus on these types of games. Let's say he has 500k subscribers, and these fans go to him to listen to his opinion on minecraft, watch him play, maybe even look at his other videos of minecraft-like games.
So let's say XxXMinecraftProXxX uploads a video about Starcraft online. And it's a positive video! He seems really into it, and he invites his fans to "Play with him" and to click the referral link. And at the bottom of the description, all the way down there past his social media links, on bottom, is 'This was paid for by Perfect World'. Some people might catch that. Many won't. Alot of times the description box will show one or two lines, in this case the referral link. So his fans, trusting XxXMinecraftProXxX's word, click this link and buy into Star Trek.
Sure, technically-
-He says it's by perfect world, in the description box
But, despite that, there is a shilling to it. It's underhanded. There is something scummy about doing that, Total Biscuit
has a great video about it.
Hell, TB in the first minute goes on about what the marketing pamphlet kotaku posted talks about-It's not made to look like an advertisement, because the youtuber is told to say something like this-
"I totally wasn't paid to say that I play this game! Come play with me!" Watch the video I linked. It goes in depth, and TB is someone who's been in situations involving paid promotions.