Seem to recall Brian Clough (I think) once saying he didn't like walls at free kicks as it can obscure the 'keepers vision, but primarily actually helps the taker in that it gives him something to try and curl the ball round. He reasoned that it was harder without a wall.
Watch almost every free kick in a similar area in almost any other game and exactly the same thing will happen. Defenders always move back as the ball is delivered. If they don't and an attacker beats the offside, they are clear through on their own.