One, I think the act of pulling something off of someone's face is a lot different from pulling something out of someone's hand. It's subtle on the surface, but crossing that kind of social boundary on a regular enough basis could be exactly the tipping point that turns public opinion against law enforcement on a broad scale.
Two, as the technology and form factor mature, it'll simply be harder to tell whether something is/contains a camera or not.
I know miniature cameras exist right now but since they only function as a camera, there's not too much utility to always having one around. Glass though has a camera but that's not it's main selling point.
Police officers are abysmally trained and are effectively immune from the law, so expect to see abuses like this continue.
See this article, and the links in it, for some examples: http://arstechnica.com/tech-policy/2012/11/jury-says-journal...
* Possibly get arrested