Probably in a similar way that Apple is a registered trademark for computers, but you wouldn’t be able to register it for fruit. Alkaline here may not even be an actual characteristic of the product, or assuming it is, it’s supposedly so distinctive that it sets the brand apart from others.

That said, it’s very unlikely that the trademark could be defended if a competitor challenged it, but maybe the goal is just to use the ® as a gimmick, to increase the perceived value by the customer, and if it became commonplace they’d just come up with some other marketing strategy…