Cops aren't lawyers, but they're still required to accurately enforce the laws. There are two scenarios here. Either:

A.) The cops in question were truly ignorant of the law as written and committed a gross act of incompetence and injustice.

or

B.) The cops knew the law but chose to arrest her anyway absent any legal justification for doing so, instead predicating their actions on an attempt to silence the press.

The latter absolutely constitutes false imprisonment - the unlawful violation of another's personal liberty consisting of detention without sufficient legal authority is the textbook definition of false arrest - and the former is unbelievable given the circumstances of her prosecution and the broader context of behavior vis-a-vis the pipeline protests. In terms of a civil matter I don't think she'd have any problems convincing a jury that there's a better than even chance that the latter scenario is true, and that's before we start getting into the realm of Constitutional violations.