Google is fairly clear about this:
http://support.google.com/webmasters/bin/answer.py?hl=en&...
"How does Google handle nofollowed links?
In general, we don't follow them. This means that Google does not transfer PageRank or anchor text across these links. Essentially, using nofollow causes us to drop the target links from our overall graph of the web. However, the target pages may still appear in our index if other sites link to them without using nofollow, or if the URLs are submitted to Google in a Sitemap. Also, it's important to note that other search engines may handle nofollow in slightly different ways."
So, if your tweet includes a link, they drop that link from the post as far as their graph is concerned. And that graph is what makes up their search.
It's not just page rank.
I think the nofollow attributes intent was fairly straight forward from the beginning: to designate links that should not be accorded attention in search results. In 2005, this was merely via PageRank. Today, it's recommendations in search results.
Why would I want to see search results that shouldn't be given weight in a search result in a search result? Explain that to me.
And this speculation for the "shared on" functionality doesn't exactly add up, either. If they're not following links with rel=nofollow, why should Google be legally or even morally responsible for storing a massive and entirely extraneous relational database, and then checking against it every time they follow a short URL from another site, just to see if it was shared on Twitter at some point?
No, not for a/@rel="nofollow", the only promise Google does is that it will not transfer pagerank credit to the link's destination[0].
This does not prevent them from using that link as an outbound in order to discover new content, nor does it prevent Google from reversing the link for the "Shared On" feature. Google specifically advocates[0] using a/@rel=nofollow for user-generated links in order to prevent rewarding spammers, Twitter's use of the attribute is not just sensible, it's necessary.
niyazpk's comment[1] looks far more sensible: "Shared On" is a specific feature for content producers and part of a special agreement and API access.
[0] http://googleblog.blogspot.com/2005/01/preventing-comment-sp...
[1] http://news.ycombinator.com/item?id=3456233