There are different types of white rap fans. A lot of them are attracted to rap for the same reasons they were attracted to hard rock or metal: the subversive nature of the genre (explicit lyrics, gangsta rap, etc). That group is a major reason why NWA blew up and took rap in the direction it went in. But there's also another group of white rap fans who just...like hip hop. Some are hardcore "real hip hop" fans, some only like rap from a specific period of time, some like virtually anything, etc. I agree the unifying thing for most of these people is that they didn't grow up with r&b and don't care for it.
If you listen to a lot of white musical journalists/fans/etc talk about James Brown you quickly realize they don't appreciate him. These are people who want to tell you all about how The Beatles changed everything about music...but will turn around and say "well James Brown had a couple good records but most of his stuff is repetitive and never evolved." Same types who will tell you MJ made generic pop, or that Prince made generic love songs. They don't respect black talent or music. These are the dudes who watched that Prince "While My Guitar Gently Weeps" video and were stunned to learn that yes, Prince is one of the greatest guitarists of all time. They know nothing about black music because deep down they're just waiting for a white artist to "do it better" (Eminem, Adele, etc).
No disrespect to white fans who appreciate black music for real, tho. I just don't think Rosenberg is one of them. "Real hip hop" is just the dikk measuring contest he decided to specialize in because it's the only one he can win. And one thing I'd note about Rosenberg...I don't get the impression he listens to much non-rap music in general. He's just a sperg who got obsessed with one thing and never let go. And the only reason he has a decent vinyl collection is because he wanted to DJ.