That's a load of crap. A shipmate and I were on shore patrol in Toulon once when we observed two guys dragging a woman into a doorway. We ran to the nearest police station which happened to be less than a block away. The "officer" there told us to mind our own business.
No, it is the law.
Often the law is not enforced.
In a case like yours, it was not.
In a case like this, though, it would be.