Why is "allegedly" a part of this report? As if the woman made up the incident is the given premise? As if it never happened?

Well, yes, indeedy, that is the basic reaction in so many men's minds about the violence women report. It a knee jerk default ASSumption which is step one in denying a problem any woman alone faces every step she takes in this "man's world".
And that's why women often don't report assaults, rapes etc... which of course is the consequence that serves this "man's world" best.

When I read about man on man violence "allegedly" is not part of the picture. Why is that? Why do men trust men when they know how often men lie about everything?

Don't shoot the messenger who is speaking in spite of the taboos and is likely to be greeted with "hatred" and "bashing" accusations even though what I'm saying is well documented.

Maher, "allegedly" is a standard word to use in any story about a crime, particularly a serious one. And the Voice uses this word frequently. Web Link

It's likely that an attacker will be named. The government will consider that man innocent until proven guilty in a court of law. Newspapers are expected to do so as well. If they state as fact something which is later disproven, the accused could sue them for libel and would likely win. It would be easy to show damage to reputation, lost wages, etc. So while it's unlikely for an accusation like this to be made up, but they're still legally required to consider the possibility. That requirement is not conditioned on the gender of the accuser.