It seems they don't want it to be mandatory because people will likely fill it with nonsense, as your already suggesting, which is worse for them than it not be

It seems they don't want it to be mandatory because people will likely fill it with nonsense, as your already suggesting, which is worse for them than it not being there. I don't know what the screen reader does, presumably tells them there is an image there. I assume they want the option to remove the post all together.