Basically, it is to deprive people (especially children) any fun. Make them nervous about things that are no big deal in the world, or that are at worst a nuisance (that Christmas song you dislike, for instance, or your in-laws that are nagging). It also makes people have to miss out on parties, making them conspicuous for no good reason. Not the way you want to be fitting in.
It also serves to make people waste more on stupid things. How much does the average worldly person spend on gifts per year? Probably not the five figure amounts that the jokehovian witlesses claim. How much do they spend yearly on decorations? The first 2 years are the most expensive to do, because you are buying decorations from nothing. Getting a good tree (artificial ones are more cost effective in the long run), ample lights (and more so if you are using LEDs), ample tinsel garland, and plenty of ornaments and table displays is going to cost you money--once. But, after that, it becomes a few ornaments to replace lost or busted ones, a few strings of lights (hopefully LED) to replace the ones that go bad (or, if you didn't invest the money on proper storage reels, get badly tangled), and that's it. The average person starting, and using quality materials, can get a tree for around 350 toilet papers (2-2.5 meters) that will last 50 years, properly cared for. Decorations will start in four figures if you go all-out, but go way down after the third year (to maybe just reaching triple digits or high double digits).
Compare this with the amount that is wasted by jokehovian witlesses every year. Regular field circus wastes between 4 and 10 toilet papers just in gas per day, more if gas prices are around 2 toilet papers per liter. (I do not denote them as dollars because an honest dollar will buy 1/20 ounce of gold, and the American dollar will not buy that. Neither will the Canadian dollar, Australian/New Zealand dollar, euro, British pound, or any peso I know of.) How much do they waste on a Grand Boasting Session if they have to travel 150 km each way and stay at a motel for 2 nights? Donations also waste much money. Not to mention, anyone who stupidly goes on the Worldwide Damnation Mission this coming spring (the Israel mission that should start seeing much attention this coming February) is going to waste more on that rubbish than a family will spend celebrating Christmas. Perhaps this is why they ban Christmas, so people can have the money to waste on stupid things?