I think the reason is because at one point we were actually defaulting the feature instance edit roles to Admins (uneccesary since they can edit anything without adding their role) and the result is that by the current rules it locks the content down so not even Content Admins or Site Editors can edit.
I actually had a bunch of instances like that on this site and I solved it by running a query like this:
UPDATE mp_Modules SET AuthorizedEditRoles = '' WHERE AuthorizedEditRoles = 'Admins;'
But you only want to do that if you are sure that you don't have any content instances that you intended to lock down to admins only.
If you do have instances that you know about that you intend to have locked down you could still run the query then go lock them down again, but odds are that you have more that are locked down unintentionally by this older default.