Twitter has been experimenting with placing users’ favourites into the timelines of their followers. The move has caused much comment and criticism. Users have complained that if they wanted their favourites to be shared in other timelines they would, in fact, have retweeted them.
'Favourite' is used in a number of ways by Twitter users – including curation and bookmarking and as a way to demonstrate agreement or support. They do not necessarily want to share this with a wider community.
Sources and more on this story from Mashable; EndGadget.