"According to an old custom, at the public defence, the Doctoral Candidate invites to the post-doctoral party the members of the public who at the urging of the Doctoral Candidate made questions or comments, but they must always decline the invitation."
Why is that? It seems that this custom is at least in Finland.
Because the poor, poor Candidate is the one footing the bill of the post-doctoral party. Besides, the party venue has usually required the Candidate to confirm the number of guests well in advance, and there might not be room or food for extras.
Why the Candidate needs to invite the inquisitive members of the public in the first place is beyond my understanding, but it probably has something to do with old rules of etiquette. I suspect that at some point in the past the invitees could actually have accept the invitation, but the etiquette has changed as doctoral candidates have become poorer and poorer :).