Yes, the U.S. Constitution protects free expression, so the U.S. government's hands are completely tied on this issue. Even if government officials personally think a site probably facilitates behavior that violates the law, actually taking a website offline, rather than targeting individuals for criminal charges only after they've violated the law, unless that website is owned and operated, perhaps, by an individual who has been found guilty of violating the law, is most likely unconstitutional. This is one of those, "private companies are forced to step up because the state's powers are limited," situations.