I was thinking about this same subject for a while. I wonder what would be brought up about "taping" the actions of others without their consent. I guess if it were as evidence if how a felony was committed, the only question would be "why do you record everything you do?" (Are you some kind of paranoid wacko? Are you a pervert trying to get free porn? etc.)
For a while in my life, I was being pulled over by the Renton P.D. every time I left my house, it was ridiculous and bordered on harassment (this is not an exaggeration, BTW). So, I thought it would be a good idea to install miniature video cameras in the windows of my car to prove to the judge what I was saying was actually happening (I was drawn on one time when I was pulled over for a license plate light being out, and I have no idea why, as there was no provokation on my part to warrent that.) I thought if I had proof of how I was being treated and spoken to, I would be able to get them to stop, but I was told that not only was that "illegal", but would be inadmissable as evidence. I don't really know, but I got to thinking "How would I explain why I had cameras in my car?"
The same question hold for the pen cams, how do you justify having one and how could you get it admitted into evidence?
BTW, tear gas can backfire, it doesn't work on everybody and if you get it on yourself, it really sucks if you're not one of the lucky ones that seem to be immune to it. (don't ask how I know this, it involves a few beers, a closed garage door and a couple of intelligence impaired friends. You do the math.)