If you haven't heard of such people, well, you've been spending too much time watching porn and you should be ashamed of yourself.
Such assertions come from ignorance. They are not from wisdom or knowledge, but just pure foolishness.
Usually, these folks will cite the following passage in Jeremiah 11 verses 2-5:
“Learn not the way of the nations,
nor be dismayed at the signs of the heavens
because the nations are dismayed at them,
for the customs of the peoples are vanity.
A tree from the forest is cut down
and worked with an axe by the hands of a craftsman.
They decorate it with silver and gold;
they fasten it with hammer and nails
so that it cannot move.
Their idols are like scarecrows in a cucumber field,
and they cannot speak;
they have to be carried,
for they cannot walk.
Do not be afraid of them,
for they cannot do evil,
neither is it in them to do good.”
Those verses certainly talks about decorating a tree and using it in an improper manner, mostly for the purposes of idolatry.
Here's the thing: the prophecy of Jeremiah was written hundreds of year before Jesus was born. So clearly, Jeremiah wasn't referring to Christmas trees. He was, as far as I can tell, talking about how people made idols to worship, probably Asherah poles.
I'm pretty sure it's not the Christmas trees that are the problem with the lack of Christ in Christmas. If you think that Christmas trees are evil and represent all that is wrong with modern Christmas, that's fine. I can't change your mind or make you understand.
But look at the situation like that of food sacrificed to idols. When Paul talked about the topic, he was referring to how some people are very insecure in their faith and that eating food that was used in the worship of false gods might cause them spiritual harm. Wanting to not let these people to be like the seed tossed in the path rather than good soil, Paul was instructing others to protect them until their roots are strong enough to withstand evil.
I suppose it is the same with people who think Christmas trees are evil. I have no problem with you if you think that way, but do not place your spiritual insecurities on me. I worship the one true God, same as you, and no Christmas tree is going to change that.