Perhaps, Steinbeck does take jabs at the hypocrisy of society needing to to shame certain things. The Mahjong games are the perfect scapegoat because its only played by the Chinese so nobody's family will be shamed.
However, Steinbeck does portray the brothels as exploitive in the case of the pimp, Mr. Edwards., who runs his prostitutes out as soon as they get too old or infirmed. He also beats them savagely. And Kathy is a vampire to her girls as well. The whores themselves are also displayed as victims of society with sordid pasts or just plain stupid in the case of Ethel. I think running an old timey brothel would be rather depressing.
On the other hand, it does fulfilled a need like the butcher, the baker and the clerk.

