Report suspicions to abuse and report false accusations as well. In fact, this is safer for you-accusing another publicly of plagiarism is libel, and is a crime. It's also a civil action that can result in stiff fines and judgments. Hurting someone's reputation is immoral as well. It is not libel if the accusation is true, however.
Truth is considered an absolute defense.
Obviously by destroying credibility and reliability, earning power is reduced. Financially rewarding cheaters is also a loss,and discourages honest, hard-working writers. Allowing thieves among us diminishes the community as a whole. Abuse has driven many good writers away. Stealing another's writing is just as bad as taking a physical possession- maybe even worse.