Hell, HackerNews leaves a cookie on your computer after you log out with some opaque blob holding who-knows-what. Users like to complain about cookies when you bring them up, but generally can't seem to bother. Including the two of us.
As stated in the article, so when you login again from the same computer, they don't have to do the whole two factor "I've never seen this computer before" text message handshake with you.