Okay let's take a step back and discuss this properly.
First of all, the First Amendment doesn't apply. This should be plainly obvious - it's about what you can *say*, not what you can *see* or *record*.
The laws that *should* apply are the ones about privacy and public decency, and the court should consider how they interact with each other here. My take on the matter is:
Public decency laws essentially require people to keep certain parts of their bodies *private*. The precise definition of which parts varies considerably over time and jurisdiction, but the typical standard in Western society is hips-to-upper-thigh for both men and women, plus most of the breasts for women.
Clothing which performs this task adequately is readily available, and (for women) usually relies on the assumption that an observer's eyes are above the subject's waist. Even short skirts are generally designed to hang down to cover the area between the thighs when the wearer sits down. Underwear is intended as a backstop, to deal with momentary malfunctions of the skirt's normal dynamics (eg. wind) and corner cases where the central assumption is invalid (small children, escalators, ladders). Some people enhance this by wearing shorts or a petticoat under their skirt.
Note that even in those corner cases, people are generally expected by society to avert their gaze. Parents of young children are to teach them that looking up skirts is wrong, and try to prevent them from doing so - in return, most people understand that small children are not malicious. Therefore, a woman wearing a skirt and normal underwear can be considered to have a "reasonable expectation of privacy" against deliberate exposure of their *underwear*, never mind what said underwear covers, to the extent that their skirt would normally keep it covered.
So now we come to the breastfeeding woman on the Tube. Here, the breast is exposed more than normal societal standards expect, although the nipple itself spends most of the time covered by the baby's head. Most sensible people would agree that this is not obscene - it does not "outrage public decency". At the same time, this woman has deliberately exposed that part of her to view, so she no longer has an expectation of privacy regarding it - until, of course, she finishes feeding her child and covers it up again. In my experience, many (not all) women try to find somewhere with fewer people to watch her - for example, near one of the shortcut passages in IKEA, rather than beside the main paths.
That is the difference between an upskirt photo and a candid photo that contains a breastfeeding mother. In one case, the photographer is violating an expectation of privacy; in the other, he is not. It is the same distinction between taking a photograph of a minor Royal on a public engagement, and taking another photo of the same person using a huge telephoto lens to penetrate the large estate she lives on.
Let's round out the discussion by illustrating two examples that *do* violate public-decency laws. The most obvious example of this would be a streaker at a sporting event, who takes off every stitch of clothing and runs out in front of a crowd of tens of thousands, specifically to cause outrage and embarrassment (not to mention disruption to the match in progress). A more nuanced example is the recent protest by a group of Finnish art students, where one of them posed as a nude model for the others - right in the middle of Tampere railway station. After a discreet interval, a policeman politely asked her to get down and get dressed - which she did, having made her point, and I understand that no charges were filed.
Biting the hand that feeds IT
