[fl0wenol]

All country's diplomatic services have a similar exempt mail/pouch system; it's a service we all extend to each other. Others leave ours alone, and we leave foreign government's alone in turn (at ports or elsewhere).

Governments are allowed and expected to keep secrets from each other; this is a way to do it out in the open.

[windows_tips]

Governments are a function of their laws. I'm not sure what you mean by, 'are allowed and expected to keep secrets from each other'.

I don't actually allow or expect my government to keep secrets. I don't think 'they' are even authorized to. If 'they' could, 'they' would not be a representative government, of, for, and by the people.

[grkvlt]

Of course governments must keep secrets. These are things like order of battle details for the military; location and other details for nuclear weapons; names and addresses of intelligence officers, informants or defectors; algorithms for decryption of foreign ciphers; and so on. However, in western democracies the secrecy is usually time limited. For example in the United Kingdom we have the 30-year rule[0] which provides for public release of confidential government documents after 30 years (and they are moving to 20 years now) which is a good compromise between transparency and the need for operational secrecy and security.

0. https://en.wikipedia.org/wiki/Thirty-year_rule