In regards to displaying decorated trees during the holiday season, I've understood quite differently from Allegheny County v. Greater Pittsburgh ACLU and from ACLU of Illinois v St. Charles. Case law has been clear that Christmas trees (or holiday trees as they're now called) are a secular, not a religious symbol, and their public display creates no constitutional conflict.
Maybe of FR lawyer will stop by, but I don't think the tree was at issue there, only the menorah, and significant only because it's display next to the menorah lent it a secular quality. As I understand it they're both considered symbols with both secular and religious characteristics.
That wasn't the issue here, though, rather the display of one and the banning of the other.