First, there's a difference between keeping the apps available through Apple's App Store squeaky clean and how consumers choose to make use of the devices they've purchased. There's very little difference between how one makes use of their phone and its features and how one makes use of one's laptop or desktop. If critics suddenly start calling for Apple to regulate how, when, and for what purposes FaceTime may be used I sure hope they're willing to demand that other companies, e.g. Dell, AT&T, Verizon, etc., do the same. Their devices and networks are used for similar, if not identical, activities.
Second, IMO I wouldn't worry about this being more than a brief flare. From TFA:
"You can have the phone on your face, or other body parts, but not both at the same time," said Teagan Presley, who acts in adult films and performs in video chats. "Most customers want the full package, and it's going to be difficult holding a phone."
Sure it's cool that you can do this on an iPhone, just like it was cool that, when the first iPhone came out, someone managed to get World of Warcraft to run on it. However, it's just not practical.