Interestingly, one of the main purposes of passports, perhaps even the reason they were invented, is to provide some guarantee that you can come back to the country that issued your passport. So the reason countries demand foreigners hold valid passports is to be able to deport them easily should the need arise.

Without passport, local authorities would typically first need to ascertain their nationality and have the relevant consulate confirm it and issue a *laissez-passer*. That's why desperate people sometimes try to get rid of their passport or hide it in the hope of defeating immediate deportation (and occasionally also to claim another nationality to get a better shot at asylum).

Therefore, historically, the main reason countries invalidate passports is to *prevent people from coming back*. For example, during a short time on the night of November 9, 1989, border guards were instructed to invalidate the passport of anybody who wanted to leave the GDR with a special stamp (they quickly gave up and just let people through). In less dramatic situations, people also often have to surrender their passport to make it more difficult to leave, e.g. following a court order.

None of this seems very relevant to Mr. Snowden's situation as he probably wants to avoid coming back to the US for the time being. However, as long as he holds onto his passport, it still would look very “valid” to any third country, no matter what US law has to say about it, as there is no legal framework to prevent people from traveling between third countries and no global database of invalid passports. I believe that might also be the reason for rules preventing entry to many countries with a soon-to-expire passport.

Several type of travel documents have also been created for refugees or stateless people who cannot get a passport from their home countries (Nansen passport, 1951 Convention travel document, 1954 Convention travel document…). They would typically be issued by the country where a person usually resides but, unlike a passport, do not imply that the holder is a citizen.

Of course, none of this really matters. At this stage, it's a safe guess that any decision regarding Mr. Snowden's fate would be done at the ministerial level, on a purely political basis.