Early on in the speech, TR drops this reference:

If Aristides is praised overmuch as just, people get tired of hearing it; and overcensure of the unjust finally and from similar reasons results in their favor. (24)

Roosevelt's giving a shout-out to classical Athenian politician Aristides the Just. (Clever political move there, Teddy.) The thing about Aristides the Just is that "the Just" was the nickname Aristides was trying to make happen, which—as we know from the universal rule of nicknames—just doesn't work. People got really tired of hearing him drop Aristides the Just everywhere.

Ol' Aristides just kinda sounded like a smug jerk.

So, people stopped thinking he was just in any sense of the word. Teddy's jumping onto this by saying if people are always calling businesses corrupt, eventually people will be like "oh, whatever," about them, too.