They're not all burgundy. If you'll notice there's a tiny strip of white at the bottom of the sock. I'm pretty sure one-colored socks result in a fine from the league which is what got Portis and ST in trouble 2 years ago.
EDIT: Come to think of it, I only remember ST wearing the crazy soccer style socks, but I'm pretty sure there was one game where Portis got fined for wearing socks that were all burgundy.