That's what the ACLU has to say about Christmas stamps, and this is why:

The US Post Office doesn't force anyone to buy Christmas stamps. They make stamps for Eid, and for Hanukkah, and for Diwali, and so on. (They don't make one for Winter Solstice yet, I don't think, but I forgive them.)

There are a ton of different stamps for people who want to avoid the holiday.

The overtly religious stamps recreate great works of art and speak to our cultural history.

And besides, though the US Postal Service is sponsored by the government and used by the government, it's not a department of the government anymore.