Now my heart says that "Up-skirting" represents a monstrous invasion of somebodies privacy. But as we are frequently told. We have extremely limited legal expectations of privacy when in public, and certainly no right not to be photographed against our will. Indeed, our legal expectation of privacy in private is fairly limited too in fact.
In many ways, using a telephoto lens to snap somebody sunbathing next to their private swimming pool in their back garden is actually a rather more serious invasion of privacy than snapping a womans knickers while she is traveling on the underground. (Unless the law has been changed, I understand that this is also quite legal and that Paps do it all the time)