Stealing attention from Jesus
While Jesus represents the religious meaning of Christmas, Satan Clause represents the secular aspects. By shifting peoples attention away from seeing Jesus as the symbol of Christmas and to worship a secular symbol instead, he turns people away from faith.
Making people more materialistic
Satan Clause doesn't reward people for good behavior with absolution for their sins and an eternity in heaven. He rewards people with materialistic gods. He wants people to not be nice for niceness itself but to be nice for their own benefit. This goes against Christian doctrine which says that you got to be nice and mean it.
When Satan Claus gifts to the poor, others don't need to
Charity is one of the seven virtues of Christianity. According to the bible, giving to those in need is important to receive salvation. But when Satan Clause gives presents to the poor, rich people will no longer feel the necessity to do the same. That means it becomes more likely that their souls will become Satan's after judgment day.