Poor wording is interfering with the conveyance of the message. Southwest lets you board the plane without an assigned seat, you then scan the plane and plop yourself down in your favorite. United lets you select a seat (on some fares) when you book the ticket or check in. You absolutely cannot board the plane and sit wherever you like. (You guys are interpreting the word "pick" as meaning both things.)
SeatGuru[1] is a godsend, especially if you are plus size or have special needs and need to choose comfortable or practical seats in advance. (Make sure to select the Seat Maps tab.)
