Not parent, but I suppose that government agencies in the US are more willing to not insist on keeping the rights. There is the idea that tax-paid work should (or must?) be available to the general public. Therefore the agencies don't have a good argument for keeping the rights anyway. As far as I know this is very much a US thing and doesn't exist elsewhere. At least where OP and I work it is - to the best of my knowledge - unheard of.

For a prime example see SQLite, which is in the public domain because its author worked as contractor for the Navy, when it was created.