I'm going to split the difference here. I actually agree with A's philosophy; it's a pet peeve of mine when a non-bulkhead first class passenger puts small bags in the overhead bin (although bigger pet peeve that just happened to me last week, the passenger in the window seat put his bag in the center section under the row in front of us, which cut into my legroom). But I would never actually ask someone to move their bag to under the seat in front of them. The most I would ever do is, if I was on a plane with 2-3 or 3-2 configuration and someone put a small bag on the deeper bin side, ask them if they minded if I moved that bag to the shallow bins on the other side. But even that I would typically leave to the flight attendant.