There is one way that an affiliate signup can get rakeback -- affiliate links dissolve if you don't make a
real money deposit within 60 days of signup.
Back when I first started online and knew nothing of rakeback, I signed up to FT through a freeroll site affiliate link. My
bankroll was divided between PS and SportsBook/Cake at the time and I didn't want to deposit at FT since I didn't care for them at first. Probably a year or so later, FT won me over and I finally deposited, and have been playing there almost exclusively ever since.
A month or so ago, assuming it was impossible but trying anyway, I sent FT an email asking about the possibility of rakeback and admitting outright that I signed up through an affiliate link. I asked if there was any way to cancel my affiliation after so much time, or to create another account, anything to get rakeback. I didn't threaten but did say I was being attracted by lucrative rakeback offers from other US friendly sites (Bodog, Carbon, etc).
They responded within 24 hours saying that I did *not* have an affiliate account, and explained that I must not have deposited within 60 days and that's why the affiliation was canceled. They said they were forwarding my account detail to RBP and they would be in touch. A day or so later RBP emailed and said I was eligible. I created my RBP account then and have been receiving RB ever since.
No idea why it's easy for some and not others. I'm not a huge baller or anything. I do multi-table 50NL and a little 100NL, and have made Silver IronMan a couple of times, but it still only amounted to 10k hands a month during a busy month.