Only because they don't know what Glass is yet; the same could be said for John Q. Public back when the first cell phone shipped with a camera.
Also, it's still possible to film covertly with a cell phone, whether it's rigging it into your clothing somehow, or simply holding it up and pretending to talk on it. Google wasn't even the first to incorporate a video recorder into eyeglasses...
Eventually? Although I've not tried them myself, a quick search of the internet pulls up numerous "spy" cameras built into all sorts of things, including eye glasses, that are virtually undetectable.
Right - and there are already much subtler devices like Memoto (http://memoto.com/) that have been created for the purpose of continuously logging photos. The obvious progression for Memoto, I'd think, is video.
They do know they are being filmed (or will know once these devices become more common), as there is a red light that flashes on the glass when it is recording. However, that could easily change in the future too - with hidden cameras that constantly record everything becoming more common.
So if I put some paint over that light, no more red light while recording. I find the argument that a light will make our privacy ok not all that convincing.
