First off the ones that a peacefully protesting have not been arrested.   North Dakota does not have "masses" of police.  Oh an they also do not have tanks, but they do have humvees and squad cars.   The only person that have been arrested are those who are tresspassing, chaining themselves to construction equipment, vandalizing, starting fires, blocking roads, assaulting the police, shooting arrows at them, and in one case a handgun.