The discrimination could occur even though the newspaper ads don't show it. For example, if you read the personal ads today, few of them will say "wife beaters need not apply" or "women with green hair need not apply." The absence of the statement means little.
That's true, especially when you consider that back in the 1800s there weren't the kind of anti-discrimination laws we have today. Companies were free to reject anyone for whatever reason and not fear retribution from the federal government.