No, they are not given away for free at checkin.

They are offered for sale at checkin if E- people want to upgrade to E+.

Once E- fills up at checkin, people are placed on the "Confirmed Awaiting Seat" list. At the checkin cutoff, people who are not checked in are dropped, and people on the CAS list are given seats. They'll fill E- first, and if they need more seats, they'll give away E+.

So long story short, it's a gamble. If you're trying to get them for free, delete your seat assignment, and don't check in too early. If you win, you get E+. If you lose, you get some random middles somewhere, separated from your companion. So I'm not sure it's worth playing this game.