The plain circle symbolizes earth, and if the tree was up, it would really symbolize peace and life. Upside down, it means death and destruction. In the sixties, some students started using the peace sign as we know it for protesting nuclear weapons. Nuclear weapons obviously bring death to Earth, so the sign was appropriate. But now the sign is understood to mean peace, love and unity when the symbolism really means the opposite.
Of course, socially, the upside down tree of life symbol has now a total connotation of positivity and peace. But this would still be more accurate:
Pretty interesting wouldn't you say!