Never understood why they didn't link this to the odometer reset. I guess some people may like to track them independently but I bet that Mazda is just sourcing two different systems and it's more work to get them to work together.
There are two odometers so it might have been a problem to reset both. I sometimes use one for a trip and one for miles since last tank of gas. I prefer to be able to reset the miles per gallon independently as I'll do that sometimes on the highway just to check out MPG at different speeds for ten miles or so but would not want to necessarily reset the odometer each time.