"I read that in the press."
Translation:J'ai lu ça dans la presse.
In this case, it's because of the grammatical function of the word. "Cela" ("or its informal counterpart, "ça") technically is a demonstrative pronoun, which doesn't act like normal direct/indirect objects in French. It's almost as though "ça" is acting like a known object. It usually has an antecedent, or a noun that a pronoun represents.