You would think that, except that Oracle can simply declare their own kernel the only one 'tested and supported' for their database. As a customer of their database, why wouldn't I use it?
Having said all that, I have no clue how Oracle is going to continue to support their kernel once they've undermined RH enough. I'm sure not many of the RH engineers want to work for oracle (apparently not many of the Sun engineers did either...). I have this impression that for Larry Ellison it's all about winning, regardless of whether he's a winner of a wasteland, rather than a large participant in a vibrant ecosystem.