If they know you are a Muslim, that's not appropriate but if they are strangers unaware of your religion, I wouldn't take offense and wouldn't correct them. Christmas has become a consumerist tradition. Otherwise why would most atheists have a tree and decorate their house? Nobody says it's for the winter season, it's for Christmas. So it turns out that Christmas is celebrated in one way or another by almost everybody in Western countries (the only exception I know of is Jehovah Witnesses because they are strict on not celebrating holidays). All in all, as a Christian, I consider this peer pressured, consumer-based, modern Christmas tradition wrong especially because Christmas is a winter solstice celebration with pagan roots. Driving around a neighborhood, you won't see a single house without a tree. Don't tell me everybody is religious. We have been conditioned to do this to spend $$$ and it's not about the Lord and Savior anymore.