Hi PD2,
the 737 isn't a 400T+ aircraft. On the larger aircraft the weight and balance (loadsheet) is usually made using standard weights and the aircraft divided into compartments. The weight (cargo and/or pax) is distributed so the the aircraft centre of gravity remains within limits.
On the smaller aircraft such as the 737/airbus it may be that the C of G is on a limit and as such blocking out or moving a single row of seats/pax will bring it back within limits. I have never seen a single pax moved to comply with limitations although I have removed a galley bar (about 90Kgs) to get a 737 below Max take off weight. Not quite the same thing but helps illustrate how critical weights can be. I know some operators will only load hold luggage in a single compartment ( allows a quicker turnround and less staff) and hence trim the aircraft by moving the other variable load(pax).
C of G can also be critical in-flight especially if the fuel in an aft tank fails to be burnt (744).
From the information I'm unsure in your example if it was a requirement for that flight. If it was either you should have been moved or off-loaded.
