I don't see where the Activity Type is referred to as a sport/activity. I'm guessing the reasoning is because motorcycling and driving are considered sports and were put into the system early on. Now we have the concept of fitness activities, and Garmin hasn't got around to adding them. Think about all of the activity types that aren't tracked. Yoga and Pilates are just two of maybe more than 100.
Doing that creates its own set of problems. Garmin needs to be able to classify activities to determine what stats to display, or how to display them. If you participate in an activity of type Ixnarg, how should Garmin display your speed/pace? Time/100m, per 25m, kph, is distance even relevant? It also makes for problems if they want to build a system to allow users to compare similar activities. I call my activity type Blarg, and you call yours Flamb, but they're actually the same activity type? Should they be allowed to be compared?