It seems a bit weird considering the last time Nintendo released something that used AA batteries was in 2006*, nowadays you'd expect lithium ion or polymer battery packs. Not sure just how much extra play time alkaline or nickel-based batteries would add, plus the fact that the built-in Joy-Con cells last 20+ hours anyway. If it was for users who didn't want to charge through the main console or those with multiple Joy-Cons that can't all be charged at once from the tablet, they'd probably be better off getting a Joy-Con dock for the same price like this one from PowerA:
*The Wii remote. If you wanted to be technical, the last product to use traditional batteries was the 3DS Circle Pad Pro from 2013, which ran off AAA batteries.