From my reading of it, the Pope is not so much worried about the ashes themselves, but rather the motivation of those doing the scattering.

If your loved one's ashes are sitting are sitting on the mantelpiece, there is no need to visit the churchyard. If you hold a little ceremony to scatter your loved one in the woods, could the next step be holding the wood sacred leading to worship of woodland spirits? If people get solace and comfort from sending ashes all over the world, will they still feel the need to turn to the Mother Church? If sending ashes into the solar system becomes popular then Rome becomes less the centre of the universe.

The Pope is worried about pantheism. He would prefer to have the ashes kept in cemeteries or churches where he has influence and control.

DC