Many Poskim (Shach, Aruch HaShulchan) hold that the whole Issur of mesira only applies to a non-fair society that would treat Jew differently but in a democratic system like the US there is no mesira as the whole Issur was made when Jews would get harsher punishments than a non-Jew.
Furthermore all Poskim agree that when someone is endangering the public (Metza'arei Harabim) and doesn't heed to instruction it is mutar to inform.
I am not a Posek but it seems as if they have some halachic basis to support themselves with to inform. If an actual Posek would say otherwise then fine. But to label them as a moser without backing is totally wrong unless they have which i am unaware of.