Many people like to know when the official 'last posting dates' for Christmas are. The Post Office would be widely castigated if they failed to disseminate the information.
Given that postal delivery staff now go to every house on their round anyway (because the Post Office delivers 'junk mail' as well as 'addressed mail') the simplest way to provide such information is to put a leaflet through everyone's door. Designing that leaflet in the form of a Christmas card costs little more than simply producing a 'flyer', so it seems to be a good idea to me.
Chris
PS: My only complaint is that the artist has failed to melt the snow around the chimneys on the houses. Surely, if it's as cold as that, the occupiers would have lit their fires?