I agree. It would be much more efficient all around if the cats would eat from the bottom up.
1) The bottom leaves turn yellow first so they get less to eat on each plant if they start at the top and the bottom leaves turn
2) As you mentioned frass does not defy gravity. If they eat the bottom leaves first, they will not have to deal with frass.
3) The flowers are at the top where the seed pods are created. If the cats at the bottom, more seed pods might develop. (My cats eat flowers too - seem to like them).
I guess trying to convince them is pointless