Hi Pieter,
Apparently ties, although they look almost identical to slurs, use a different type of machinery internally to produce their shape, and the \shape functions only partially work. I do not know why the code is different, but I suppose lilypond 'just grew' and that's how it is. Most people end up having to use TieColumn techniques in this case.
Others more learned than I in lilypond internals will no doubt be able to explain this better.
Ties are not really very perfect in lilypond, and there was an effort to fix them up, but I understand that project came to end end due to time and resource matters on the part of the developer.
Andrew