It's far more complicated than that. Without a witness and/or evidence, such a case probably wouldn't make it to court.

The underlying fact is: if the two witnesses (the insulted and the insulting) give contradicting statements, the court has to decide whom to believe. Usually, as stated above, such cases get thrown out of the window. However, if they end up in court, it is indeed easier for the insulted party to claim the court's ear. This is not due to law or anything, but mere tactics: the insulting- accused - party cannot be tried for lying to the court.
The accusing party however faces prosecution, if caught lying to the court.

This causes many judges to believe the plaintiff - especially as there has to be some reason for the case ending up in court (plaintiff a policeman, known history of similar incidents, etc); usually a case without 3rd party witness doesn't make it that far.