I'm sure that is Michael Newdow's next project. Sue the federal gov't for having a Christmas national holiday. He's sooooooooo offended.
That is the ultimate answer. Right now, they're just fighting the symptoms. As long as Christmas is a federal holiday, there is nothing wrong with having Christmas parades. Trying to abolish individual Christmas parades and displays while leaving the underlying Christmas holiday intact will only keep the fight going.
I don't think they'll ever be successful in removing Christmas as a national holiday.