Copyright's raison d'etre
It's important to bear in mind the justification for copyright restrictions: in order to provide creators with a reward for creating work, so I can make money out of writing novels, developing software, composing music or whatever. None of that applies to Parliament, which is not supposed to be in the business of, er, business: it's funded by our taxes in order to legislate on our behalf, not to create commercial products!
The US government has the right idea here: we pay for it through our taxes, so the copyright belongs to the public (i.e. public domain) not to the individual bit of our government we paid to do the work for us. If they license something from a commercial entity, that's different - but anything we pay for, we should own.