According to the brief details in the report the charge was for having the taser without a permit, not the actual tasing. If you consent to being tased I can't see how it should be a crime. Cops get tased as part of their taser training. Us non-cops may have any number of reasons to want to be tased.
The surprising part to me is that there is a state where you need a permit to own a taser. WTF?
Also, I suspect that since the guy "bought several tasers at a truck stop" these are not projectile-firing tasers but the more run-of-the mill "stun guns" which use direct contact -- apparently upon her butt.