so would putting up a christmas tree in the white house endorse a religion?
This is where they start down the path where we can declare them crazy.
When the government says, "Everybody come down to the fire station and get your tree - anybody not in compliance will get fined $5 per day until you put your tree up." THAT's establishment.
When the mayor of NYC is of a Christian denomination and he puts up a Christmas (yeah, I said it) tree in his official residence that's him freely practicing his religion. Just as a Jewish mayor puts up a Menorah, etc.
The White House is POTUS and family's home while they are President and First Family.
Just as the community ought to be able to put up a nativity at a fire station or a tree in the square of the town - the rights of the minority should be protected, but they should not dictate observances of the majority. I cannot see what is so bad, here.