Maybe I'm paranoid thinking back to my flying my instructor was strict about weight and balance not that I carried anything as a student. But always being aware of it is a big part of flying or at least when learning to fly.
My instructor even made me secure my pen when I was flying nothing loose in the cabin.
For a small plane, the limits are smaller, naturally. But half a ton here and there is something an airliner can deal with.
For the pen, there might be other reasons - a dropped pen might slide under the rudder pedal and block it, or interfere with other things in the cabin.
