This is by design. It's helped greatly increase the amount of people downloading the actual full-size image by surfacing the panel with the download option. This portion of users was probably saving whatever size was currently in view (rarely the full res) with the OS right-click before that change was made.

Blocking the stock OS action would be getting in the way of actions other than downloading.

See T75999 for details (and T76001 and T75963 for alternative solutions which would look less ugly but would probably be less effective in deterring users from unintentionally downloading a resized version of the image).