Citizen's arrests are perfectly legal within certain guidelines, and thus would not provide sufficient grist for the press mill in and of themselves.
There is no justification for sharing this information.
Then do not provide your name to law enforcement when calling in a tip.
Legal or not, it would be a breaking story in the media. Giving water to illegals is also legal, but Minutemen giving water to illegals is a news story. Minutemen doing anything is news.