What you were told is both right and wrong. Many buildings in Germany had to be rebuilt after WW2. These were often build according to similar plans, with the mailboxes right behind the entry door. Some sort of tradition, probably, or it just fit the architect's plans. There's not always room for external mailboxes in some layouts. Even some modern buildings follow this plan, there is no outside mailbox. But not because of malicious camera probing persons, most of these were built in the 1950ies (think bigger cameras). Usually some housewife was always there, to open the doors for the postal or communal workers (trashbins were usually in the backyards, and had to be moved through the house on trash day). So times changed, and the fulltime housewifes also had to work to secure a living for the family, and nobody would be at home to open the door. So people tended to leave a front door key with the -then- monopolist Deutsche Post delivery person to ensure their letters got delivered. This was ok for a time, but the monopoly broke and DHL deliveries are no longer done by the same person, they shift their delivery area according to some strange loadbalancing scheme.
So, tl;dr: Most of what you were told is probably outdated, and most of what you angrily interpret into an unconvenient situation is wrong. Hope this helps to clarify, and please try to be a little bit more open to other, non-malicious thoughts.