I promised I'd share my thoughts... I see two very different ways of thinking about this. 1- exactly what Sagittarius said. It's not about his pleasure (although that's probably a happy byproduct) or receiving feedback. It's just her and the penis, no distractions... whatever she wants to do is what she's gonna do because it's about her at the moment. 2- she's dreadfully uncomfortable and wants no chance of making eye contact. No awkward faces. No "why is he looking at me like that? Am I doing this right? Does he not like it? What's that look mean?" She's just trying to get it over with and not let her distaste be so apparent.
I fall into camp 1. My friend (who hates it, thanks to a horrible first experience with a guy jamming her head down) is in camp 2. From a personal perspective, it's also more comfortable to lay across his stomach than his legs in most cases.
I will admit that I used to be in camp 2 and always always ALWAYS had my eyes closed because I didn't want the chance of making eye contact. Ugh, I hated that because I was insecure. As I became more comfortable with myself and my sexuality, I still didn't like being watched, but did want to see what I was working with, so I'd have my eyes open and just avoid looking at his face. Now I have no reservations/hangups. Sometimes, I want the feedback and turn on of seeing the guy's face. Other times, it's not at all about him... it's for me. I think he's usually glad to be along for the ride, but that's not the point of the excursion.
Thanks for those who gave their input.