...Btw, yes she was an American woman indeed, but I am 100% sure that she was an exception to the rule, and that her behaviour would not be seen as acceptable in the USA as well!!
I won't speculate on which nations are the worst in this respect, but unfortunately, I don't think such behavior is that uncommon here. I would ask "may I use your pen?", or "do you have a pen I can use?" before picking up even a disposable ballpoint, and I would take no for an answer. Many, probably most, other people would, as well. Unfortunately, still others, even if reasonably well behaved in most respects, seem to have a blind spot when it comes to pens. They seem to think that any pen lying out in plain view is for anybody to use, and can even get indignant if you refuse to let them use it, or stop them from grabbing it.
I had this problem, though, before I started carrying fountain pens, when all that I had was some nicer than average ballpoints and rollerballs. I learned not to leave my pens where they could be grabbed easily and learned to say no. I'm not going to let the poor behavior of others stop me from enjoying my nicer pens. Luckily, I don't have to worry so much where I work, as people know how I am about my pens; it would only be the occasional visitor who might be a problem.
If I do feel like lending someone a pen, I generally have one ballpoint with me, even if I'm not at work, but training the general public to carry their own pens is a pet project of mine.