Most of the people that take your bags at curbside checkin are not airline employees. They are an independant company. And it is up to them to then transfer your bag to the respective airlines. Maybe things have changed in the last couple of years, but my husband will not use curbside. He'd rather pay $1 to get one of those cart things and bring it to the airline directly...
Don't want to sound like a "downer".... that has just been our experience.