Why is Montag's capture depicted on television even though he has escaped in "Fahrenheit 451"?
Since Montag has ingested a solution which changes his body chemistry so that the Mechanical Hound can no longer track him, the authorities cannot capture him. It is unthinkable for the government to reveal itself as fallible, however, so the troops focus on an innocent and anonymous scapegoat, making it appear that they have indeed overwhelmed the renegade Montag. Television viewers do not know the difference, however, and unquestioningly believe what they are shown and told - they see a man being apprehended and are told it is Montag; accustomed to accepting whatever is presented to them by the government-controlled media, it does not occur to them to doubt what is presented as truth. Even though the powers-that-be have been defeated in this instance, the public does not know it. In their minds, Montag has met his just reward, and the government remains infallible and supreme (Part 3).
The firemen needed a neat ending to the day's events. It was more important to have a dead body than to have the right person. It wouldn't do for the criminal to have outwitted the authorities and to have escaped; that would look bad. So the camera identified an innocent man as Montag and they had a nice, neat ending. Thus, it is possible for the real Montag to escape to the countryside.