I am an administrator in my organization.
I need to find a regular user to test this with. But, I'm being told here that what you experience is currently normal.
However, I swear that yesterday when I was looking at my content, through a regular user's login, they couldn't change anything. Maybe I was wrong.
If it is true that regular AGOL users can re-enable editing on a web map that they don't own, then that is a problem. All the more reason to roll your own web editng maps.
I would have thought the fact that users who don't own content shouldn't get access to change map settings of someone else's content would have been the norm. That would also fix the problem.
However, what if... you create a lame duck user account. That user saves a copy of your web editing map. Then you assign this map created by the lame duck user to a web app (ESRI Basic Viewer) and let your other read only users see that web app?