A player doesn't need to go down to be awarded a penalty according to the laws of the game. There are 10 infringements that are punishable by a direct free kick or penalty:
•kicks or attempts to kick an opponent •trips or attempts to trip an opponent •jumps at an opponent •charges an opponent •strikes or attempts to strike an opponent •pushes an opponent •tackles an opponent •holds an opponent •spits at an opponent •handles the ball deliberately
Walcott was held and partly spun, thus impeding his advance towards goal. It was a penalty, clear as day.