From what I have read - it isn't just a case of someone going into another room for 15 minutes while the baby is in a bumbo on the kitchen counter.
It is the parent turns their back to get something out of the cupboard and out goes the baby. It can happen really fast so supervising doesn't ensure it doesn't happen. Who doesn't bend down or turn their back for a second? The seatbelt is a good addition. It isn't the whole bumbo falling off the counter. It is the child getting out of the bumbo and falling off.