What specific law of Logic am I breaching?
modus ponens
Ps. Btw, I'm under the impression that 'Santa' in Western countries is saint Nicholas, who is a real person.
Yes. But usually when you say his name you mean a guy who brings you present. It doesn't matter whether his based off a real person. There might also have been a person named Jesus. Still that does not mean he did things the Bible claims he did.
But let me turn that question a bit on its head: If someone believed in fairies, would you engage in a 5-day conversation with him, trying to convince him otherwise?
Probably not. But a belief in fairies is not a religion.