Stupid yes, but one of the members of the Trump transition team - who is also the son of our new National Security Advisor, believes this nonsense and perpetuates it. Like any conspiracy theory, it cannot be proven "untrue" ever, to the conspiracy faithful, so their logic is that so long it is not proven untrue, it might be true but since you can never prove such wild theories untrue (any proof proffered is shot down or said to be manufactured) then the conspiracy is never unproven. A man armed with an AR-15 went to the pizza shop, shot up the place, searched it, didn't find any children or child sex ring or secret rooms in the basement. You would think that would settle the issue. But he isn't sure, so maybe it is true. Welcome to the bizarro-world of conspiracy theories!
The goal of whoever started this rumor is to shut the pizza shop down - put the owner out of business and punish him for his political beliefs. No doubt the owner offered his opinion online and someone didn't like his opinion. So they tracked him down and started this rumor. Trolls use other techniques, such as "Swatting" - calling the police in your jurisdiction and claiming a hostage situation has taken place at your home. The SWAT team shows up at your house and busts down the door. So far no one has been killed yet, but this illustrates how powerful we've let the Internet become. And don't think it can't happen to you, just because it hasn't happened.... yet.
This is the brave new world we have of "free speech" on the Internet.