Origin
According to a fake news story published on the self-described "entertainment" web site Daily Finesser, the U.S. Congress has passed a law authorizing citizens to shoot and kill "suspicious" or "malicious" clowns on sight:
Authorities Authorize The Public To Legally Shoot And Kill Suspicious Clowns
Nationwide – After the mysterious nationwide apparition of clowns, a new law has been passed by congress to authorize civilians and police officers, to shoot and kill any malicious clown they see alone in the streets.
Hundreds of clowns have been appearing across the nation, many of them threatening children and adults, chasing them. Many incidents have been caught on camera and have gone viral on social media. Authorities addressed the situation and assured the public that they were conducting investigations to find out the motive of these clowns. However, it seems that things have gone overboard after it was announced that anyone will have the legal right to shoot and kill any clown they see alone on the streets, saying “If you don’t want to be shot, don’t be a clown."
But while it's true that the country began to go slightly "clown crazy" in September 2016 due to the rapid spread of rumors, hoaxes, and junk news about creepy clown sightings all across the U.S., no such law has been enacted — or could be enacted, even if they wanted to — by Congress.
As stated in a disclaimer at the bottom of every page on the web site, "Daily Finesser is for entertainment purposes only," which, come to think of it, is supposed to be true of clowns, as well.