They have to use the term "alleged" because they have not yet been convicted, as stated above, as this can get them out of a slander/libel lawsuit and still be protected by the 1st amendment freedom of press. This is problematic though, as when a newscaster says "an alleged drug dealer" or an "alleged rapist", most people in public do not hear the word "alleged" as much as they hear the offense. So it is very easy in this country to be convicted in the media before such a trial has taken place.
Later, if a person is found innocent, do they run another news story clearing it all up? Unlikely. As a teacher this scares me quite a bit. I have never committed a major crime (other than speeding, etc.) and never would, but I am one student accusation away from having a reputation forever tarnished, and having the word "alleged" in front of the accusation won't save that reputation. In countries like Canada (where I believe this is still the case), the media cannot report on crimes with a name of a suspect unless that person gets convicted.