An "overweight" bag is considered to be over 50 lbs.; if you suspect your bag of being near that limit, just weigh it on the scale provided before you hurriedly click past the corresponding screen at the kiosk. Even if you aren't paying for the extra weight, the agent needs to tag the bag with one of the "heavy" markers so the handlers know to treat it differently.

This doesn't excuse bad behavior; if you caught this woman's name, please alert someone regarding it. But there are valid non-financial reasons they need to know if the bag is over 50 lbs.

EDIT: Looks like fastair has also pointed out some other good reasons the airlines request this information.