Interesting to read your reaction .Scott if you, holding a valid ticket, are offloaded to allow a family who haven't a valid additional ticket insisting on using an extra seat for a car seat for an infant. I'm sure you would be very understanding of the situation and walk away with a smile on your face.
Definitely agree that these issues should be resolved at check-in ideally, or at the gate at the latest. With internet check-in being used by so many I think the gate staff are becoming more important as the check-in process is by passed by many passengers these days