This is likely to be a matter of policy rather than law
That is, it’s not likely there is a law prohibiting it but it is highly likely that the person’s training and their employer’s policy on the matter is that they must complete a ticket once they start it.
It’s a pretty universal anti-corruption measure - it prevents the situation where they are writing the ticket, the owner appears and offers them half the value of the fine in cash to stop: well, they can’t stop so they can’t be tempted by the bribe. Of course, the bribe can be offered before they start but, when fighting corruption, you minimise the opportunities rather than eliminate them.
I know that police and rangers (private people can’t issue tickets) in new-south-wales are so restricted.