CarolineS and WorkingDogz are quite correct.
Unfortunately, many misinformed-negligent-ignorant-amteurish-inattentive-unschooled-careless-irresponsible people, including criminal gangs and outlaws, got "pitbulls" and Rottweilers for a variety of wrong reasons (macho symbol, guard and attack, illegal dog fighting, protect illegal activities, and so forth), and the rest is history! These two, otherwise magnificent breeds, have been tremendously hurt in the hands of all kinds of irresponsible people.
Sadly enough, the facts speak for themselves. It really hurt us, responsible dog owners as well as ethical breeders, a lot. It is painful to see how wrongly our breed is been portrayed and labeled... but there is a reason for it:
IRRESPONSIBLE PEOPLE! 
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:(