Memes

Any big & powerful dog with a strong bite like the pit bull has the capacity to seriously harm & potentially kill a person, and since you can't count on every pit bull owner to responsibly train their pets, they do become a liability when in public. Pit bulls are also a popular breed in the illegal dogfighting scene, so violently dangerous dogs that have been bred to be violently dangerous are guaranteed to exist.

Even so, it's rather unfair to treat every single pit bull like a menace when non-aggressive pit bulls that are affectionate towards strangers are not uncommon. Laws requiring big dogs to be muzzled should suffice; banning the entire breed from public (or, in some places, from even existing) seems excessive to me.

Edit: ...well, at least in this comment, most of my points still stand. I should add that pit bulls are not only popular for dogfighting, but also a favorite of criminals in general, so much so that their demand is actively driving the breed to be even more violently dangerous than ever before. This has become such a serious problem that unaggressive pit bulls are nowadays unlikely to be purebred.

I guess it's still unfair to treat every single pit bull (or, rather, every dog that resembles a pit bull) like a menace, but it'd also be unfair to blame anyone for treating them as such, so long as breeders continue to select for stronger, more aggressive, more dangerous traits.