They have always released new sets in this manner. They don't need new sets to encourage sales in December, you need new products to encourage purchasing when people aren't already looking to buy.
Also, I think too much weight is being put into the no discounting exclusives policy. Sure, people may not be buying as many exclusives, but they're still spending the money, just on different stuff. If someone has a $2,000 budget, they probably didn't stop buying sets because they can't buy exclusives, they just put that same money into other stuff they could get at a discount. Either way, most people weren't buying their exclusives from TLG directly since Amazon usually had bigger discounts on them.