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.
The term alleged means that a person has not yet been convicted of a crime, just accused of a crime. In the United States, it is law that a person receive a fair trial. Until the criminal allegations have been proven then that person is considered innocent by law. This where the the phrase, "innocent until proven guilty" comes in.
This is also related to presumption of innocence which states:
a fundamental protection for a person accused of a crime, which requires the prosecution to prove its case against the defendant beyond a reasonable doubt. This is opposite from the criminal law in many countries, where the accused is considered guilty until he/she proves his/her innocence or the government completely fails to prove its case.
Thank goodness in this great country one is "innocent until proven guilty". You hear the word alleged because any criminal complaint is an allegation or accusation until the truth is determined by the "fact finder", judge and jury. Many procedures must be adhered to before one can be found guilty by a jury of his peers. The body of rules pertaining to this process is called Criminal Procedure.
Anyone can accuse others of criminal activity. You can say anything about anyone, but you must prove your suspicions and accusations in a court of law. Further, law enforcement personnel must have probable cause to make an arrest(more than a whim). Realize though that even an arrest means absolutely nothing in regards to the defendants guilt or innocence. Innocent people are arrested every single day and sometimes guilty people are never arrested.
There are a couple of reasons for this.
First of all, here in the United States, a person is innocent until (or unless) proven guilty. For that reason, until something has been proved in court, you cannot really say for sure (legally speaking) that they have committed the crime.
Second, the media outlets need to be careful so that they do not get sued for libel or slander. If they say unequivocally that someone did a particular crime, they are saying that they know for sure that this happened. But when they used the word "alleged" they cover themselves because they are just saying that someone else says the person has committed the crime.