Since many of the flight attendants don't have kids they don't realize that the Bjorn is a carrier and not a harness vest restraint. So since they don't know what it really is they will error on the side of having you remove the child since vest restraints are not allowed for taxi, takeoff and landing. The are only allowed during cruise. The FAA really doesn't address Bjorn and similiar carriers since they aren't safety devices. So they are a "gray" area.
Kim