In terms of planned or not-exactly-emergency c/s happening more frequently with celebs...I'm sure liability plays a role if this really is the case. Just like mamas who are themselves doctors, or lawyers, or when the pregnancy is the first/only child after a mama's long battle with infertility...OBs tend to see these mamas as having more to lose, or at least, more to press charges over and they go with the procedure that will provide them with the best legal defense in case of accident.
However, I'm sure the hospital stats, state birth climate, regional standards of care, length of time since last birth (or c/s, I'm pretty sure Brit wouldn't have been "allowed" to vbac even if she wanted to considering the relatively short time between her births), and even the track record of the OB/care provider in charge have a bigger impact on the c/s rate than personal wishes of the mamas involved. Celeb or not, a mama giving birth in a state with a high c/s rate or in a hospital with a high rate or with a care provider who isn't familiar with or supportive of vaginal birth is certainly at higher risk for a c/s no matter what she has planned.
So while I think it's perfectly fair to ask if celebs have c/s more frequently (and why? Legal concerns on the part of the OB? Job based needs of the mama involved? General poor health due to infection, lifestyle, weight concerns, medical history) , I think the assumption that women in the media are more likely to request or get a c/s than the mainstream mama round the corner is a little unfair.