It's about what's being leveled. When you do Netherwing dailies, you're leveling a specific character's Netherwing reputation. The mount at the end is the reward for the leveling. You can't throw 5 characters on the same account at Shadowmoon Valley and walk away with a Netherdrake in a day. If Character A is two dailies short of exalted, Character B on the same account can't finish it off for her.
With pet battles, you're leveling the pet, and that progress is shared. If 5 characters on the same account all took a Tiny Sporebat and battled it, well, that pet is probably now 25. If Character A got your Tiny Sporebat to level 23, Character B can make the final push to 25.
The pet battle dailies are account wide because the progress is account wide, and reputation gains are not. The reward for getting exalted may be shared, but the act of getting there wasn't.