The reason being that only one location(the Chi-Chi's) being the source of the infection. If a field of green onions in Mexico had been infected, common sense would tell you that many other restaurants would have been infected since all those green onions in that field did not go to one restaurant.
It is clearly not a Chi Chi's employee. In the past several months there have been 3 distinct outbreaks of Hepatitis A linked to green onions. One was in GA/NC, another in TN, and the third in PA. In all cases the virus was isolted and partially sequenced. The sequencing showed that all isolates from a given outbreak were identical and the three diferent sets of isolates were closely related to each other. A case control analysis indicated that the source was greeen onions, and the genetics of the virus indicated that although each strain was unique, each was closely related to a strain commonly found in Mexico as well as travelers returning from Mexico.
At the end of 1998, the same thing happened in Ohio. 43 patrons contracted Hepatitis A, it was traced to green onions, and the strain was closely related to a common Mexican strain.
These data suggest that the contamination is probably in Mexico, but very localized, so by the time the onions are served in restaurants, the contamination is fairly concentrated.
The Chi Chi's outbreak was large because they have a high volume (11,000 meals in Oct served at the one retaurant) and the vast majority of those infected at mild salsa during a three day period at the beginning of Oct.
However, some of the outliers (those affected very early or very late), may indicated a chronic low level infection that usually goes undetected (people do get hepatitis A, but either they don't know they are affected or have no idea that the infection came from food eaten a month earlier).
The chance that the Chi Chi's outbreak was linked to deliberate contamination by a Chi Chi's empolyee is very close to zero.