Why do people with tourettes say outrageous things?

It is a verbal tic. People with tourette syndrome can have both motor and verbal tics. Coprolalia, the term for the verbal utterances are the result of the same causes as motor tics and occur in approximately 15% of individuals with tourette syndrome.
Only some do. People with this disorder get the label because they have both a chronic motor and oral/verbal tic or spontaneous event. For the majority this would involve something as simple as throat clearing or cough.Many learn to cover their verbal utterance as a cough. Vulgar words occur in some, but these are just what is available in their brain, not something they can select/control.