UUA is not really a 'change' is it? It's not like you have changed your mind about something and want different days or different hotels.
For those I guess it makes sense to have a 24hr cooling off period and charge people outside of that, but UUA may not even be available when you book but become available later, so there is no way to do it at booking time. Charging would then be a punitive charge on using Avios which are an advertised benefit. As crazy8534 says I suspect there is a culture at BAH that uses certain words and style that may just have carried over into an UUA query when it isn't appropriate.