If not, what keeps them steady? I can see nothing in the design that would stop them moving side to side.
https://www.ebay.co.uk/itm/10L-8L-Cycling-Bicycle-Bike-Saddle-Seat-Tail-Rear-Bag-MTB-Storage-Pouch-Carrier/263216965554?ssPageName=STRK:MEBIDX:IT&var=562210880479&_trksid=p2055119.m1438.l2649
https://www.ebay.co.uk/itm/10L-8L-Cycling-Bicycle-Bike-Saddle-Seat-Tail-Rear-Bag-MTB-Storage-Pouch-Carrier/263216965554?ssPageName=STRK:MEBIDX:IT&var=562210880479&_trksid=p2055119.m1438.l2649