More likely to be smacked carelessly into things as you move around, though.

Apple has always, always had price delineation across the range. They've also probably sat down and looked at expected use-cases for their Watch product range, the Sport doesn't receive a Sapphire display which could be price motivated as well but as the above article says the Sapphire is likely to be more effected by drops or impacts. If you're engaged in a sport there's a higher chance of catching the screen than if you're just wandering around, which makes sense to be have a screen that's more impact resistant.