I agree that single examples do not justify squat but at some point the number of examples grows large enough to become empirical eveidence (at least in your mind). How many Jewish people (or any other distinct group) would have to try shortchange you (or pick any other behavioral phenomenon) before you would start to believe there is a positive correlation between Jewish people and shortchangers? How many times would it take after you counted your money twice (or made sure you had legal recourse, etc) whenever you dealt with a Jewsish person? As the frequency of the conjecture increases the confidence in it also increases. That's human nature even though it is shoddy logic.
Stereotyping is a funny beast and people tend to get all bent out of shape about it, which I find funny. One thing I have found is Jewish people tend to react wildy to statements made against their people or Israel. You don't see Asians getting riled up when somebody says 'All chinese are good at math

'. And for those of you who don't believe Asians are good at math, head to your nearest university and hit up the math department, preferably in the grad school.