You could also use fixated:

def 3: (Psychoanalysis) to develop a fixation; suffer an arrest in one's emotional or sexual development.

Fixation, n.
def 5: a preoccupation with one subject, issue, etc.; obsession
_{definitions from dictionary.com}

Normally this takes on as its preposition, so your example would be

The mainstream media are fixated on reporting gossip about politicians and celebrities.

You could also use fixated:

fixated, adj.
def 1: so interested in someone or something that you do not pay attention to anything else
_{definition from macmillandictionary.com}

fixate, v.
def 3: (Psychoanalysis) to develop a fixation; suffer an arrest in one's emotional or sexual development.

Fixation, n.
def 5: a preoccupation with one subject, issue, etc.; obsession
_{definitions from dictionary.com}

Since this is referring to a psychological disorder, it is definitely negative in tone.

Normally fixate takes on as its preposition, so your example would be

The mainstream media are fixated on reporting gossip about politicians and celebrities.