Because if you don't, some bright spark will find the thing that says you must and file a suit against your company saying you lost the name they changed their mind about, even if they hadn't actually done so. I'd do as above (and in fact have), and tell them it's required to send, but they can ignore it.
Not sure how it would be seen legally, or in fact programmatically by WHMCS, but maybe in these cases you could cancel it in the system. As far as I recall, that stops the reminders.