I'm sorry but I really don't understand your post. On one hand, you say you're not saying that people should put up their trees according to the advent church year and yet you don't understand why people would take down their trees before Epiphany.
I think the main thing here is to understand that people observe Christmas or the season however they wish and there is no need to understand someone else's reasons for taking down their tree according to your beliefs. Even in the Christian world there are many different calendar dates for Christmas, Easter, etc.
Personally, I could care less when people put up their tree or take it down. To each his/her own.