Fair enough, but to be clear, they are all designed to be plugged into a wall receptacle and be stationary. Twist locks are for connections where it is critical that the plug not become disconnected.
Dryer plugs are supposed to be a very tight fit and are not meant for frequent disconnection. So the question is, are the “mobile” chargers meant to be used regularly from place to place, and if yes, the provision to do so and make it UL certified is probably the twistlock.
I’m not saying of a certainty that it is a UL requirement, but it seems quite likely. Any high amp 240v or 3 phase equipment that I’ve own and that was connected via non-stationary or fixed connection, required a twistlock to pass electrical inspection. Also, RV receptacles are stationary. A mobile or portable charger has an inherent uncontrolled application. So, I can see that it is quite plausible that this is a UL requirement.