Am I overreacting in being bothered by this?
Yes, you are.
As far as "the system" is concerned, its done its job. You wanted seats allocated together, it gave you seats allocated together. What you do with those seats is your problem.
Dread to think the sort of panic you must get into when you get one of those four-seater tables allocated on a train and the computer puts your family round the table in a random order.
