My understanding is that if journalists don't say that things are alleged then they open themselves up to lawsuits - or at least that's the rationale I've seen presented for things like this in the past. It fits the meaning of the word, too: "asserted to be true or to exist".
Today's journalists overuse this word. It's only relevant when speaking of the "alleged" suspect or the suspect's "alleged" actions when the person's participation or their actions are not yet proven. Once proven, the word alleged or allegedly is inappropriate, and it's also inappropriate to describe a crime or characteristics of a crime as it wrongly casts doubt on the victim.
See this for a clear explanation of how this term is often used in error:
