I'm not surprised that most of these incidents have happened when someone tried to cancel the first day or two of a reservation. As mentioned by others, the obvious issue is that the shorter reservation may no longer comply with the original rate/stay requirements.

I'm also sure that wise Bruce and others know that it is sometimes possible to game the reservation system to obtain a lower rate on a high capacity night or availability on a sold-out night by making a reservation for a longer stay that includes the desired dates. In many cases, subsequently cancelling the unneeded nights will preserve the discounts and availability. The cited stray cases of rate recalculation may be an attempt to address this practice.

In the case of adding a night, there should be no expectation or entitlement to the existing reserved rate. The rates then currently available are fairly what should be offered.