I don't know of any other applications which force you to log into them before you can use them. Usually, the assumption is that if someone has access to your user account, they have access to all of your stuff.
Indeed, if someone has access to your user account they don't need the GUI - they can just go and edit Syncthing's config directly, or access your files.
The only point of the password is to prevent other users on the same machine who are logged in at the same time from accessing Syncthing, or to prevent users from accessing Syncthing over the network (if you've enabled that). Neither of those apply to the desktop application.