Well, she answered your question. She's used to doing it in public and therefore has to be quiet. She's still quiet because that's what she's used to. Just try to get her to relax and let loose.
I once had a boyfriend that didn't make noise during sex. No grunting, moaning, dirty talk...
I just don't get it. If someone wants to spend THEIR money on something kinda expensive why does everyone call them spoiled or wasteful? I mean, if I work and pay for it myself then what's the big deal? Just because I don't want to buy all of my clothes at Walmart doesn't make me a bad person or...