That's the robots nofollow, we're talking about a/@rel=nofollow which google advertises quite differently: http://googleblog.blogspot.com/2005/01/preventing-comment-sp... and the microformats standardization[0] stuck very carefully to "provide no additional weight or ranking to target" period. Not that crawlers can't follow that link for new content, and not that crawlers can't reverse that link for "shared on" features, just that this precise link won't add to the pagerank of the target.
The thing is, if Google were to say “x was shared by y on twitter”, that is precisely providing some additional weight to the result. Human weight, admittedly, though I believe this also factors into the weight on the page. So it makes sense that twitter links would also not be included in this sharing stuff either.
[0] http://microformats.org/wiki/rel-nofollow