This is something I've noticed recently since airlines have started allowing an extra "personal" bag as part of the free cabin baggage allowance. I checked in for a flight last week with just my laptop bag and the attendant automatically put one of those yellow tags on it that says it must be stored under the seat in front of you. They reiterate this as an announcement in the cabin, specifically stating that you are not allowed put it in the overhead bins if it has one of the tags.
AIBU to think this is quite unfair? I intentionally pack light to make travel easier and to guarantee I'll be able to use the leg room under my seat. Why should I be cheated out of my space in the overhead bins (and risk a potential blood clot due to lack of leg room) just because of the hoarders who need a carry on suitcase and another personal bag? Needless to say I promptly removed the tag and stored my appropriate sized bag in the overhead bin.
(As an aside, if everyone packed lightly the plane would use slightly less fuel and therefore be more environmentally friendly)