I have a 2012 Prius Plug-In with a separate charge port door. It is completely manual with no electric motor to open and close. You can open the gas door remotely from the interior, but not the charge port door. The charge port door requires a push to open and then you have to close it manually. The charge port door sits absolutely flush with the surrounding body panels when shut.
It appears that Tesla has added complexity to fit and finish with an automated, electric motor operated, remotely activated charge port door to give the appearance of a "luxury" vehicle. Just imagine the criticism Tesla would have endured had they put a manually operated charge port door like the Prius on a "$60,000 car", not realizing that a 2012 Prius Plug-In had a $40,000 sticker price with about 1% of the technology of a 2018 model 3. The outcry of model 3 owners especially in cold winter climates that had to endure manually opening/closing the charge port door would have been deafening. What do people want, simple manual operation with fewer problems or complicated automation with more problems? Remember, that model 3 automated charge door will be on the $35,000 base car too. The model 3 is far from a "luxury" car when priced in the middle of medium priced cars - those high priced technology features and options do add up with additional potential problems!