They're actually called the "Outlaws", since Sweepers were originally (Or at least they were popularly) known for hunting players who picked on low levels or were generally annoying. PK PKers, in a sense. However now that most of the Sweepers, or at least those who are True Sweepers and not just vignettes, are working with the Ebony Strykers they haven't really been hunting too many Outlaws. Largely due to most of those Outlaws joining up with the Ebony Strykers, and the efforts of Dark Wraith and Moon Rider somehow managing to keep the [url=http://tvtropes.org/pmwiki/pmwiki.php/Main/WhiteKnighting?from=Main.WhiteKnight]White Knights[/url] from fighting with all the [url=http://tvtropes.org/pmwiki/pmwiki.php/Main/Griefer]Griefers[/url]. Technically everyone could call themselves a Sweeper, but unless you've been given a job by an Eraser, it's all just smoke. There is purposely no solid structure amongst the Sweepers to ensure that not just anyone could claim to be a Sweeper and diverge the secrets of the Erasers, who control them. And even True Sweepers like Donny have their hands too bloodied to risk coming out and revealing Eraser secrets, which even a regular True Sweeper like Donny wouldn't know. Bullet Hell is also a true Sweeper with some skeletons in her closet, so she won't risk trying to ignore a direct order from an Eraser as long as they can black mail her.