Your scenario isn't exactly what happened here. Rather, how would you feel if your friend asked you to link 10 Justin Bieber songs in your Python tutorial?
As I said, I am asking my friend to link my Python's and I link his C++.
I can treat him a lunch when I get some ad revenue later. Or just a bro hug.
I understand it may not actually be showing what is happening here, but there is some gray area I want to explore.
I can also be a celebrity and every week I feature a couple links showing what my friends and my other celebrity friends are doing or writing or making. I may or may not get $$. Maybe just to be friendly (think Youtube, how channel owners help other owners...) Will that violate Google's ToS? I am famous and so helping a few people out shouldn't hurt, right?
the problem here is they are/were promoting a campaign in which they were asking you to specifically place "beiber" links on your blog, so that they get better page ranks in google.
you and you're buddy (or even a celebrity helping out their friend for free) are trying to share a particular link with your blogs viewers, and that single instance will probably not have a crazy effect on google page rank
RG is trying to game the system by getting bieber backlinks on a bunch of blogs in trade for a tweet...thats against googles rules