Here is what it is like to those on the inside of the door:
The children got up about an hour earlier than normal. Obviously, they are anxious to start opening up the presents under the tree (and most children are not going to want to sleep in on Christmas morning). After a light breakfast, they are about to get to the presents when the doorbell goes off. The witlesses are at the door, and they successfully waste 15 minutes of the parents' time while the children are getting impatient to open their damn presents. Or, it is around 11, and they are on their way to Grandma's when the witlesses show up and waste 15 minutes of their parent's time. This pxxxes off the children, who are bound to be pxxxed off the remainder of the day. At which point, they are as likely to act up because of this disruption. That could turn what started out being a merry Christmas into a dreary Christmas.
From the outside, it isn't quite as serious. However, there are quite a few parents that do not like the idea of the witlesses intruding on family time. True, this is often the only time they will get many householders home. But, what happens when they tell the witlesses that the children are getting ready to open their presents, want to play with them in peace, or they are expecting company? Usually, the witlesses will mark it down as "busy", but sometimes they are idiots enough that they just have to have the contact information. Or, worse, yet, they get into a discussion that, every time it seems to be ending, another issue comes up and wastes another 15 minutes of the children's present-opening or Grandma-visiting time.
To me, this is one major reason why it is more than a nuisance to be out in field circus on Christmas Day. Other holidays, they might be simply cooking something and be able to spare 15 minutes. (Though, it would be the parents that get pxxxed off if their holiday dish is ruined). Most of the time, the witlesses at the door is about like a telemarketer call. However, on Christmas (and, if they are out in field circus in late afternoon, Halloween when children want to get trick or treating as soon as possible), it is downright offensive. I find it offensive to deliberately delay children's opening their presents on Christmas morning or starting trick or treating on Halloween. I would rather see them get up early, get a light breakfast, open their presents, and pig out at lunch when they are likely to be really hungry. Or, have a light supper at 4:30 PM or so on Halloween so they can get a nice, early start trick or treating. And then the witlesses have to show up to ruin it.