Dana is the promoter not the owner, he can't force champions to take fights. He can run the event and cancel the events(though he has to pay) he can choose not to work with fighters but he doesn't own them.
It's the difference between an organization (NFL, NBA, WWE) and a promotion. Dana has to come to a contractual terms with the fighters for every fight.
As for GSP's reasoning, clearly it's money. Johny isn't the draw Diaz is, Diaz has a fan base 10 years in the making, two years ago 75% of you didn't give a crap out Hendricks. I feel like if they had an winnable fight for Diaz in the top ten this doesn't happen. Diaz was given the two guys he could beat, Penn and Condit he looked good enough to get a title shot GSP/Dana and the UFC are jumping on it.
"There are no strings on me"