Why do private member variables behaved like protected in the same module when creating deriving class?

[Timon Gehr]

On 27.10.18 05:20, Mike Parker wrote:
> If used to be that even if you moved the declaration of B to another 
> module, then main would still have access to y through b. That was 
> deemed to be a bug an is now deprecated.

That seems to make no sense at all (clearly main can access y though b 
by just implicitly casting to a superclass reference). Why was this done?