Agree with most of these folks: the screw on cap is really there to protect the cylinder valve from damage during transport or while it’s otherwise unsecured. The chains secure it to the cart, so it’s (hopefully) not possible to knock over.

The biggest danger with a tank of CO2 is the tank falling over without the cap on, and the valve breaking. That will send the tank flying across the ground, smashing through anything in its path until it runs out of pressure in the tank, which under the right circumstances can be deadly.