The title of the show and the constant use of profanity is, according to Penn Jillette, a "legal maneuver." Because while calling people liars, frauds, charlatans and hacks could be construed as slander, calling someone a "fucker" and say that they're selling "bullshit" is okay.
Penn has stated that those they ask to be on the show to interview know in advance what the name of the show is, what kind of show it is and even generally knowing what side Penn and Teller will be on. Not only this, the interview subjects are even given past shows, with the exception of season one.
While we never see Teller speak, his voice is occasionally heard while his face is obscured or while he is off-camera.
The show was created to honor the spirit of master magician Harry Houdini, who devoted the latter part of his life to debunking some widely accepted (as he called it) "Humbug." The episode on profanity featured the title "Humbug" instead of "Bullshit!"