I think as a society, we are conditioned to accept certain behaviours, which are learned by observation.
Take smoking for example. Every person you see somkes in the same way - the way they light a cigarette, hold it, smoke it, stub it out - all identical, because it is learned behaviour, and people are comfortable with a predictable set of behaviours.
Look at a blind person - the way they sit, the way they move their heads to catch sounds more clearly - all appear odd to us because they have never learned the standard way of doing these things, which we learn by observation, and again, standard behaviours make us feel secure.
The issue with Tourettes is the sheer unpredictability of sufferers. It means they can move or shout without warning, all of which upsets our notion of learned responses, and makes us feel insecure, so although 'fear' is probably too strong a term, I think Sqad is right, people find it unsettling, as they do any behaviour that is different from normal - and I use that word in the strictest sense.