I agree that popularity and on-topicness are orthogonal, but in this case I think the question as asked was reasonably enough on topic. There is perhaps a valid issue due to all answers would necessarily be speculation, but that doesn't take away from the physicsness.
Technically the question is how a trick was done, which you can argue is off topic if you really want to on the grounds of speculation. However, with the minor modification of asking whether the physics of magnetism or electrostatics could be applied to create the observed effect, it would be clearly on topic. This was what the asker was asking, and what Floris answered in a excellent way.
Think of how this question would have been perceived if indeed electric or magnetic fields turned out to be plasible explanations. I doubt there would have been much objection. It seems unfair that we are closing a honest question only because the answer turned out not to require much physics. However, the asker didn't know that, this being essentially his question. Those that closed the question are basically requiring the asker to have known the answer to the question and thereby know it was off topic.
I think the question was therefore reasonable. If my answer, which didn't contain any real physics, were the only one after a couple of days or so, I could see the point in closing the question and have it eventually be automatically purged from the site. However, we ended up with a great discussion of the physics that showed how the observed phenomenon could not be the result of magnetic and electric fields. That seems just as valid to me as the same physics analisys in a different situation showing that it is possible. A negative result is still a valid and useful result. It's still good physics analisys whether the result shows something is or isn't possible.
What if the question instead showed a photograph of one of those pens being suspended in air in a special pen holder, and the question asked how that was done? I doubt anyone would feel that was off topic. Floris' answer would be almost the same, except this time the numbers would show that magnetic fields could indeed explain the observed phenomena.