What would you prefer they have done? Clubbed her over the head with a nightstick?

There's a little thing called a Force Matrix in any reasonable PD. Stun guns aren't lethal force, and they don't generally carry the risk of permanent injury (like batons and pain-compliance techniques). In some cases, that means they can be used for active noncompliance.

*Can* doesn't mean *should*, but that's probably why the article is worded the way it is. The investigation is still ongoing, after all.