I don't know, and I suspect that the task of tracking down definitively and conclusively exactly what the conditions are when I can and cannot read things would be lengthy and relatively unrewarding.
I have better things to do than play "hunt the foible" on Google.
In my experience, the conditions are pretty much as described.
If Google knows you have an account because of tracking devices, they require you to log in because you are more salable.
If you're anonymous, you're more salable if they just show you the content.
In so far as I recall, originally G+ wouldn't show content without javascript turned on - i.e. the price of content was to allow Google to track you more closely.
Then again, I only access my gmail from it's own dedicated VM and noscript prevents GoogleApis and Analytics by default.
Do you think it possible that Google is requiring you to log in because it can identify you via typical tracking mechanisms?