Are they guilty of plagiarizing the intro to "Stairway to Heaven" or not? I say no. Were they "influenced" by the Spirit song? Probably, but I wouldn't call it copyright infringement.
http://www.cnn.com/2016/04/12/enter...stairway-heaven/?iid=ob_lockedrail_bottomlist
http://www.cnn.com/2016/04/12/enter...stairway-heaven/?iid=ob_lockedrail_bottomlist