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 godsmaterial goods. But Jesus told us, craving materialistic wealth is wrong (Matthew 6:19-21). People should be nice for niceness itself, not just for their own benefit.
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. Rich people are already a main source of souls for Satan (Marc 10:23-25). But by discouraging rich people further to be charitable, it becomes even more likely that their souls will become Satan's after judgment day.