The charging port cover, as far as I know, isn't a security mechanism, and it opening is just a matter of user convenience. Having an easy protocol to open it means that a third-party charger can open it automatically to be inserted more easily, so that seems fine.
Even if they did require some sort of signed message from the charger, since anyone can buy a "wall connector" charger for home the private key would already be out there (unless a visitor had to pair their per-car key before they could use your charger? That sounds like bad ux)
There's already some people in the twitter comments talking about this as a security issue, so I'll respond do that too.
I guess you can sort of say this is a security issue (like I'm sure with enough work you could short the battery or such)... but jamming such a device into a car would be about as obvious as shoving a brick through the window, keying the paint, or any of a number of other physical attacks. This is the same sort of security issue as the security issue of "car's glass windows can be broken by a determined malicious individual". We don't run around with a fear of "cars aren't secure because a malicious person could put a brick through the glass", and a fear of "a malicious person could open the charge port and do something" is way further down the list of any reasonable person's concerns imo.