I would assume you can just DDoS either the central cerver or the entire charging location, not individual charging stalls. And in the latter case, who knows which out of many cars present near the charging location did it.
Keep in mind that a lot of supercharger stations are contained within much larger parking lots/garages. More specifically, they are simply just random stretches of parking spots in a garage, but with supercharger stalls installed.
As a real example, 3 closest supercharging stations to where I live are: a large office building multilevel garage, Trader Joe's multilevel garage, and a giant multilevel garage at a mall. Each one can fit bajillion cars (maybe not the trader joe's one, as it only has 2 or 3 floors, with about 25 parking spots on each).
If one of those cars did it once, then well, it's a spare change for you. Doesn't matter if the grand scheme of things.
If one of those cars is involved in its third DoS happening in the last quarter, you may be able to alert the authorities while it's still charging.
You can fail safe with your failsafes. You can react differently based on different kinds of failures. You can put limits to how generous you want your failsafe system to be. You can have an independent killswitch for your failsafe mechanism. You just need to put some effort into designing your system's failure modes.
You may also find out that DoSing the charging station in order to use it for free simply doesn't happen in the wild and that you can safely afford to only react once you notice it actually happening. That's why you pay all these security engineers that are able to reasonably assess these risks, you know.
You'd also have to consider threat models that include attackers other than Tesla owners trying to get free charges. Eg, Tesla competitors looking to cost them money, people that have twitter beef with Elon, ransom/extortion attacks, teens in it for the lulz, etc.
You always have to consider threat models like these. If you don't, you're not doing your job well.
If you're trying to imply that considering these threat models is a reason not to come up with a proper fail-safe system, then the very same argument applies to creating these charging stations and letting people use them in the first place.
