So what is the big deal? The deal is that almost none of them are Christians.
Again, what is the big deal, you may ask? It is a big deal, because this is not a politician or a government expressing a seasons greetings - it is real people, ordinary people celebrating a festival that is not theirs, with as much fervour as celebrating a festival that is theirs. Many of them have visited churches, attended mass.
There is no contradiction there - from a Hindu or even from an Indic religion perspective. From an Indic/Hindu perspective, any ritual is the way to a common god - or more correctly a common truth. Note that there is no heaven or hell.
It is worth a thought if you find a similar practice in non-Indic religions - true pluralism in action. In monotheism there is very little space for syncreticism - because they believe that only their path will lead one to salvation - otherwise you are destined for hell.
There are arguments that this is because they are exposed to it through school and through learning in Christian schools - these arguments are only partially valid.
The ease with which we adapt, meld and synthesize is worth a thought. But saying this will not win you any friends.
Note: I had titled the post as Syncreticism but realized that I meant Pluralism