Yea..they are at their weight limit. Cabin bags count as part of the weight of a passenger. As soon as you put it in the pit, it now counts as ADDITIONAL checked bag weight. That's just the way that the regulations are written. They use an average weight for each person which includes coats and carry ons (it has seasonal and regional variations) and an average weight for pit bags, with modified numbers for heavy items such as sports equipment.

This is common on express carriers where weight limits are hit with greater frequency.