24th century United Earth France isn't exactly the same thing as 20th century real life France. United Earth would need to have a unified law system, for no other reason than the ease and speed of travel. It's likely that weapon ownership isn't heavily restricted on Earth at this time, especially since TNG era phasers are multi-purpose survival tools in addition to weapons. (And it's probable that America would insist its second amendment be integrated into the United Earth Consitution before signing on.)
Do remember that this is a civilization that lets schoolchildren play with antimatter for their science projects. In a setting where any teenager could easily make a multi-megaton bomb just from school supplies, I don't think they're worried too much about weapons control.
When was the last time there was a murder on Earth of all places? TNG-era Earth is a crime-free utopia. The police aren't equipped to handle murders any more than modern police are equipped to handle dinosaurs. They're a thing of the distant past, and generally a waste of resources training people for.
Laris and Zhaban probably disposed of the bodies, as well. Phasers have a vaporize setting, so it's pretty trivial to do. A murder investigation would require establishing that those Romulans actually existed in the first place.
