He says it is to make him feel like his mother is part of the wedding which, under the circumstances is a completely understandable sentiment. Must have been awful for him to lose her at such a young age and difficult now to get married without her being there. Not sure I would fancy that ring if I were the soon to be Princess Catherine though, given the way Chas and Diana's marriage turned out.
That said, she was hardly in a position to knock it back I suppose.