If you are logged into google, google knows who you are. It will produce results from your Google+ network even if that information is not public.

It can't do that for twitter. It doesn't know your twitter ID, nor does it have access to your twitter network. It can crawl all the twitter pages just as it crawls the rest of the web but that may not be as relevant as the search results from your network (and hence rightly may be ranked lower).

If twitter wants to appear alongside your Google+ results, they would have to open up their twitter social graph to google (or any other search engine) so that google knows who you are and can produce results accordingly.

As a user I think this is good. I don't like the idea of my data being held hostage by sites.