A lot of lemons were being brought to the scales whole and long dead ( a day old or more ). Therefore no good to eat, and being wasted after weighing.
Ensuring only trunked fish would be weighed, meant that even if the angler didn't want the fish afterwards, someone else could still eat it and it wouldn't be wasted.
If you are worried about the weight calculation accuracy its perfectly acceptable to bring the head/gut/fins/etc back in a plastic bag and have that weighed with the trunk.