There are decisions about which reasonable people can disagree. If every such decision risked going to jail for assault, kidnapping, or false imprisonment then no police would be willing perform arrests. We use (or should use) criminal prosecution for extreme cases (a police officer who shoots an unarmed suspect for no good reason) but use the normal processes of workplace discipline (for which the worst that can happen is that you lose your job) for questionable arrests (like arresting someone for "drug paraphernalia" for having a straw).