I feel this particular situation (such as the OP's) often occurs when the flight is almost completely boarded. At that time, the gate-check luggage have all been taken care off. And someone boards at the very end,....the FA"s think its just more work to get the gate check people. So, they just find the nearest, smallest bag and ask that person to stove it under the seat.

I strongly feel that if that small laptop bag is the only piece being carried, then, that person should be allowed overhead space for his/her one item.