It was, because well, he lost. Much les controversial at the time, than either of Canelo's results against Golovkin, to give a recent example. Walcott had a much better opportunity than Schmeling, Farr, Godoy, Conn etc. They would all have licked their lips, at the opportunity of fighting that version of Louis!
65-70% of watchers said Walcott won, Louis and ref thought so too. The internet certainly helps when it comes to GGG vs Canelo controversy.
There are accounts disputing this. By his word he agreed with the verdict the ref had calling him a good man and he wouldn’t question his judgment.