(this isn't directed at you, Rockaholic, but your commented provides a jumping-off point. When I say "you", I don't mean you, personally)

It's not about assuming you share the same belief system, it's about sharing their own with you. If that person is concentrated on Christmas as their celebration, it seems only natural to me to wish others a merry Christmas. It's also a fairly innocuous method of sharing that belief. People tend to bristle when Christians proclaim their belief in Christ, whether they knock on the door or stand on street corners preaching: "Oh geez, another Bible-thumper telling me I'm going to hell." I'm not a Bible-thumper, nor am I a door-knocker, but I will do what I can to represent my faith. And if that means wishing folks a merry Christmas instead of happy holidays, then that's what I'll do. If I offend someone or they reply with greetings representing another faith, then that opens up an opportunity for me to talk to them about it and for us to share our perspectives peacefully. It's not about shoving it down your throat, it's about sharing what we know to be true.

Edited to add: That being said, I understand that there are extremists out there, as there are in every religion, wanting to shove Christ down your throats. I don't condone said behaviour, nor am I trying to endorse or excuse it.