I'm disappointed but not surprised that United doesn't give us any reason other than courtesy to cancel a Basic Economy trip we won't use: If United gave any incentive someone would figure out how game it.
It doesn't matter to UA if she cancels in advance. Their algorithm has already factored in no-shows in setting inventory and selling seats. Further, there will be a standby waitlist expressly in anticipation of no-shows, which she will become when she fails to meet the check-in cut-off. UA benefits not at all from an advance cancel, and needs provide no incentive. And the notion that you should receive some sort of free perk for violating the CoC is just plain bizarre.